Update on Korean Ciphers

I have been wondering how Hangul script can be enciphered. While mentioning some possibilities in "Substitution Cipher for Hangul", I have not seen historical instances. Then, Torbjörn Andersson from Sweden, who saw my first blogpost, told me of actual examples, in which a straighforward solution of separately representing consonants and vowels forming Hangul characters was actually used. This first clue allowed me to do some search and I now uploaded a new article, Korean Cryptography.

Mirabeau's Undecoded Letter (1787)

In addition to the Spanish decryption of a French cipher mentioned yesterday, Desenclos and Clinet (2025) presents their own decryption of a letter of Francis I to Christophe Richer, mbassador to Denmark (21 January 1547). The feat was achieved in February 2025 by Ioana Ionescu under the supervision of Paul Zimmermann and was completed and verified by Camille Desenclos with the key found in the archives (BnF NAF 8431, f.182). By comparing the ciphertext and plaintext, it can be seen that the cipher has homophones, double letters, nulls, and symbols for short words or titles ("que", "je", "faict", "et", "le roy de Danemarck", "Escossois", "le roy d'Angleterre", etc.).

They also present an undecoded letter of Mirabeau (the last image of the article). They give a date of 12 April 1787, but the cleartext seems to be "X. en reponse au n^o 3. le 12 aout". Anyway, if this is 1787, it is shortly after he came back in January 1587 from a stay in the Prussian court, where he had been sent on a mission by the Foreign Minister Vergennes. (His trip is recorded in Histoire secrete de la cour de Berlin (Internet Archive; translation: The Secret History oof the Court of Berlin (Internet Archive).)

The ciphertext is transcribed below:

549 1450 623 506 71 611 1296 61 57 1146 713 884 1017 878 556 655 846 703 991 984 791 806 1023 511 723 482 814 467 1090 1030 687 705 1151 426 934 1059 1002 99 1231 882 738 1199 1071

Mirabeau is said to have devised, during imprisonment when young, a cipher for representing a letter with two figures (LANAKI), but it would have been for personal use.

The short specimen above seems to be a full-fledged diplomatic code and has a number as high as 1450, which is higher than the size of typical French codes at the time (see my blogpost). It is wondered whether the code really had about 1500 entries (rather than using high numbers as nulls). If the above is all we've got, cryptanalysis would be impossible. But code numbers from the following page are faintly visible. If more pages (actually, it has to be many more pages) are available, it may be an interesting challenge for codebreakers.

Spanich Decryption of Claude Blatier's letter (1582)

One of many interesting pieces described in Desenclos and Clinet (2025) mentioned the other day is the Spanish decryption of a letter of Claude Blatier in Tournai to Henry III (23 October 1582). The arrangement of the decrypted key with borders is very similar to that in the decryption of the Duke of Anjou's cipher I quoted from BNE in "French ciphers during the Reigns of Charles IX and Henry III". Probably Blatier's cipher was also decrypted by the same Spanish codebreaker, Luis de la Cerda.

My article mentions a short ciphertext of a Claude Blatier du Belloy, a French agent in the Low Countries, with what seems to be the plaintext in the margin. I was hoping to establish the mapping between the cipher symbols and the plaintext, but the Spanish key does not seem to match. So this short specimen remains a riddle. (Some letters of S^r Blatier in the Low Countries (1584) in BnF fr.16127 (f.196 ff.) do not include cipher.)

Ciphers in the Archives of French Foreign Ministry

Desenclos and Clinet (2025) I mentioned yesterday not only provides many images but also the authors take care to cite primary sources to support their statements. Especially, materials from the archives of the Foreign Ministry are new to me.

Among others, I was excited to see the image of the original cipher for the French plenipotentiaries for the Treaty of Ryswick in 1697. It is the same as the cipher used by them in a first talk in Ryswick in 1694, of which I posted a reconstruction in "French Ciphers during the Reign of Louis XIV". (It will be seen that the reconstruction on my page needs some correction: the inflectional endings "ais" and "ait" should be "ois" and "oit" according to the convention at the time.)

Conversely, I posted the original cipher between Henry IV and the Landgrave of Hesse-Cassel in "French Ciphers during the Reign of Henry IV of France" but the specimen from BnF fr.15920, f.162 (Gallica) was not in my list.

These are only two of the many materials included in this article. It is a treasure trove for students of French historical ciphers.

Earliest French Ciphers (1507, 1513, 1521)

Introduction of ciphers in France was later than in Italy or Spain.

While absence of extant ciphertext does not mean there was no cipher, the pursuit of the earliest specimen is not without merit. As of 2018, the oldest specimen was from 1526. In 2023, I thought specimens from ca.1520-1521 might be the oldest (blog post; identified as Antoine Duprat's reports (1521) in Camille Desenclos and George Lasry, "Cryptanalytic and historical challenges with unidentified encrypted documents from the early modern era" (HistoCrypt 2025), 2.2).

But I now see a specimen from 1513 had already been reported in Camille Desenclos (2021), "Le premier essor de la cryptographie en France (1510-1630)" (HAL). It is a letter in cipher from Louis de Solliès to Florimond Robertet, dated 8 July 1513 (BnF Dupuy 261, f.121).

And now, an even earlier instance is given in Camille Desenclos and Sarah Clinet (2025), "Diplomatie et gestion du secret" (Bibliothèque diplomatique numérique (BDN)). It is Robert de Gramont to Georges d'Amboise (ca. 1507) (BnF fr.2930, f.189 (catalogue info)).

Heavy-Null Cipher Challenge

Inspired by Henry of Navarre's cipher posted the other day, I created a challenge cryptogram.

It is a monoalphabetic substitution cipher and the plaintext is English. The catch is that it includes a large number of nulls. I know once nulls are removed, modern computers would solve it in an instant. But I suppose trying every possible combination of 26 significant symbol numbers is not computationally feasible.

If someone succeeds in solving this, please let me know how this can be broken.

79 41 80 68 50 26 82 76 61 80 92 66 13 18 52 56 17 46 71 25 64 79 98 74 18 93 18 51 26 81 67 75 35 37 71 73 46 74 52 29 52 26 16 95 85 65 30 14 31 24 26 35 82 78 66 37 95 28 85 69 25 39 26 36 15 51 52 35 64 41 19 41 69 50 82 14 28 31 32 26 85 78 95 65 55 40 64 46 27 44 90 11 27 64 26 37 37 26 47 35 37 55 41 92 61 38 36 15 13 59 73 41 99 80 69 46 83 79 35 39 24 91 44 26 68 59 79 37 69 73 93 79 68 49 82 49 87 18 94 36 45 59 90 16 66 66 83 33 28 26 44 49 18 26 18 68 29 35 73 81 85 33 90 16 28 95 69 26 91 97 35 91 57 43 56 20 30 23 58 52 26 44 73 80 66 55 10 37 55 44 89 55 83 92 32 22 91 91 44 40 13 79 16 79 29 98 62 99 87 27 17 24 28 53 63 58 35 46 82 55 48 25 85 74 45 52 18 68 96 11 16 15 45 42 44 74 35 19 44 39 47 37 68 35 56 66 73 55 22 42 80 34 84 25 90 44 14 32 89 13 98 92 37 46 68 49 43 24 15 34 35 39 65 44 47 66 71 64 26 40 35 76 99 96 96 92 53 25 26 37 46 15 69 15 40 92 35 46 11 26 31 86 94 75 84 85 84 52 31 66 92 55 77 49 26 87 79 82 74 37 70 19 69 55 34 83 23 73 93 28 44 29 34 91 90 28 90 87 35 56 87 40 49 81 67 15 34 46 25 93 64 63 34 33 46 51 87 26 25 67 49 48 96 61 59 64 66 80 66 73 23 64 52 91 93 26 31 62 11 97 28 44 34 45 56 46 20 65 27 67 34 32 37 83 22 46 68 14 52 40 49 10 43 38 18 35 66 43 70 26 34 46 60 32 36 68 90 76 68 80 38 82 96 22 94 11 67 41 96 55 67 67 61 37 48 71 47 71 10 55 92 27 24 18 67 40 24 92 93 66 28 87 23 23 26 58 94 94 83 84 15 65 61 49 65 27 98 45 46 99 30 92 14 80 98 52 18 67 47 82 31 66 41 22 85 48 25 55 81 17 27 42 82 41 50 71 26 44 15 13 60 30 71 46 51 81 26 30 26 34 44 37 47 26 25 22 53 75 66 90 69 26 34 91 36 42 49 84 34 55 35 46 84 36 36 55 68 90 44 73 13 98 25

(To insert the null symbols, I used a Perl script generated by AI. Of course, I verified that the above can be decrypted correctly. Randomness based on the rand() function may not be ideal but I believe it served the purpose. On the other hand, when I first asked AI to create a random sequence, it failed my expectation.)

Heavy Use of Nulls in Henry of Navarre's Cipher (1586)

More than half of the symbols are nulls in a letter (1586) from Henry of Navarre, later Henry IV of France. I have observed several French cipher letters employing many nulls, but this is by far the heaviest use of nulls.

I uploaded a new article, "Heavy Use of Nulls in Henry of Navarre's Cipher (1586)" to report this. 

 

Ciphering Errors of Elizabeth Stuart, Queen of Bohemia

Elizabeth Stuart, Queen of Bohemia, tried a polyalphabetic cipher in her correspondence with Thomas Roe, as described in "Ciphers in Early Stuart England before the Civil War". It switches Caesar substitution tables between words or word fragments. The key in Arabic figure placed before the word (fragment) indicates the shift in the Caesar cipher (counting from the plaintext itself). Thus, when the key is 4, "a" is enciphered as "d".

This cipher is called "Key 1 Roe" in Nadine Akkerman (ed.), The Correspondence of Elizabeth Stuart, Queen of Bohemia, Volume II (2011), p.1062, according to which only the keys 4, 5, 6, and 7 were used (obviously because larger shifts would make the counting too cumbersome).

Nadine Akkerman and Pete Langman (2024), Spycraft: Tricks and Tools of the Dangerous Trade from Elizabeth I to the Restoration provides specific examples (p.388, n.134). A typical ciphertext is:

I have had newes from K.[Arundell] 50.[Emperor] 6nfzn.[hath] 4kmyhq4.[giuen-] 4lmp.[him] 5mnw[hir] last 4dkwzhu[agswer]

("K" and "50" are nomenclature elements.) 

This short specimen includes two enciphering errors: "hir" should be "his" and "agswer" should be "answer". The first error is a simple counting error, while the latter error arose because counting was in the opposite direction. That is, for the plaintext letter N, the fourth letter afterward, Q, should be the ciphertext letter, instead of which the ciphertext actually includes K, the fourth letter forward.

Similar errors due to opposite shifting were made when "electorat" was enciphered as "4bobz5yswey" (p.387, n.132). Here, E is incorrectly enciphered as B (instead of H) and C is incorrectly rendered Z (instead of F).

Akkerman and Lagman considers this kind of errors suggests use of a cipher wheel. That is, this can be readily explained as "reading the plaintext on the inner wheel and the ciphertext on the outer, that is, the 'wrong' way around". (You can see this by switching the plaintext and ciphertext lines in the above image.)

I think, however, counting in the opposite direction is more likely. The fact that only four shifts (4, 5, 6, 7) were used seems to show that Elizabeth did not have a table or a wheel at hand when working on the cipher. She was counting for enciphering each letter.