Several well-known unsolved ciphers such as the Voynich Manuscript, Beale ciphers, Dorabella cipher, etc. have been attracting attention of the worldwide cryptologic community (see, e.g., Elonka's List of Famous Unsolved Codes and Ciphers) but historical archives (and other publications) contain many other pieces written in codes and ciphers that remain unsolved. The following lists some of such pieces in the hope that only a fraction of the efforts directed to major cryptologic puzzles might contribute to the solution of these small puzzles.
(Recently broken ciphers are also listed.)
The reader is kindly asked to provide information if he/she knows or attains decipherment of any of these.
BnF Clair. 327, ff.279-280 is an unsigned letter of 11 April 1528 to Seigneur Garbino. Use of superscript digits is also seen in Venetian ciphers at the time. The base symbol may indicate the initial of the word/syllable represented. Several letters of Hieronimo Ranzo (BnF fr.2988, f.2, f.9; BnF fr.3019, f.73) and a memoire in Italian dated Madrid, 11 April 1528 (BnF fr.3022, f.44), all undeciphered, are also in a similar cipher with superscript digits. See another article.
BnF fr.2988 (Gallica), f.21 ff., contains many undeciphered letters in symbol cipher. From other letters in the same volume, I listed this under the heading "Italian? Cipher (1520s?)" in March 2021. It turned out to be letters of Mary, Queen of Scots. Undeciphered ciphertexts in the same cipher are also found in BnF fr.20506, BnF 500 de Colbert 470, and BnF fr.3158. These results were published in George Lasry, Norbert Biermann, Satoshi Tomokiyo (2023), "Deciphering Mary Stuart’s Lost Letters from 1578-1584", Cryptologia. See another article.
The cipher of BnF fr.2988, f.1, was broken by Torbjörn Andersson from Sweden in 2017 (Cipherbrain). Unexpectedly, the plaintext turned out to be English. Another letter in the same cipher is found in BnF fr.20506. Given that many letters in the same volume now turned out to be from Mary, Queen of Scots, the recipient of this letter may be Mary. See another article.
Apart from the pieces listed below, I found that some unsolved ciphertexts in SP53 correspond to plaintexts in the same volume (see another article). Thus, there is some possibility that the plaintext for the following pieces may also be found.
This is a letter from Walsingham to his codebreaker John Somers (21 July 1581) and encloses a letter in cipher found in Mary's intercepted letter.
The enclosure (no.50I) is entirely in cipher, not deciphered. Here is my transcription.
Of the five ciphertexts of no.28, this is the only one not marked "Decifred." (The other four are Fontenay to Mary, Fontenay to Nau, and Denis to Mary.) Addressed to "ff". Here is my transcription.
I solved this in 2023 (see another article). It begins "Monsieur? Morgan? est pri[s]onnie[r] en la Bastille q?ue la nuict du premier dimenche de ...." Thus, this is information from France.
Marked "Decifred", but the contemporary decipherment is probably lost. Some symbols look like those of the Mary-Fontenay Cipher. Here is my transcription.
Anonymous letter in cipher to Mr. Tempest, an English priest resident at Paris. Cleartext lines are in French. Endorsed by Phelippes. Not deciphered. Here is my transcription.
Another anonymous letter in cipher in the same handwriting with no. 78 (copyist's?), addressed to Doctor Barret, President of the English seminary at Rheims. Endorsed by Phelippes. Here is my transcription.
"To Fulgeame" (known in various spelling: Fuljambe, Fullgham, or Foulgiam). Marked "Decifred 21 Julye 1586" but the contemporary decipherment appears to be lost. My transcription is not ready.
SP53/22 is a collection of ciphers related to Mary, Queen of Scots, in which f.52 is a short undeciphered ciphertext endorsed "Cifer with[?] Spanish Spye:" "Spanish spy".
The verso of the cipher key of f.40 also has a short undeciphered ciphertext.
There are some undeciphered ciphertext in letters to Walsingham. One is from Robert Bowes (1583) and another is from William Davison (1584). See another article (which provides my transcriptions).These ciphers look simpler than some of Mary's ciphers, and may be easy for those who can read the cleartext parts of the letters.
King Ferdinand's letter to Garcilaso de la Vega, his ambassador in Rome, of 31 August 1498 was deciphered by Parisi (2004) (pdf; p.115) but the last ten lines are in a different cipher and is left undeciphered.
Galende Diaz (1994), cited in another article, mentions a cipher of Garcilaso de la Vega preserved in the archives.
You can post your comments or solution here.
BnF Espagnol 318 (Gallica) includes undeciphered letters. Ff.5-6 and f.116 employ known ciphers but the following three are in unknown ciphers:
f.122, no.95 A letter of 8 January 1497 (An approximate solution of this was provided by George Lasry in 2022 (see another article).)
f.120-121, no.94 Viceroy of Sicily to Ferdinand, 27 April 1503
f.118, no.93 (p.448 of pdf) Lorenzo Suarez to Ferdinand and Isabella, Venice, 24 February 1504
See another article.
You can post your comments or solution here.
PTR,LEG,54,DOC.13 is a letter entirely in cipher, not deciphered. It is filed with letters from 1504. The last line may be in a different cipher. There are many repetitive patterns (like "16e", "8ρ", "ogθ+", etc.) and it does not seem to be a very complicated system.
For contemporary Spanish ciphers, see another article.
You can post your comments or solution here.
I called for a solution of an undeciphered letter (1509) of Catherine of Aragon to her father, King Ferdinand at MTC3 in 2018. To my pleasant surprise, it was immediately solved by Victor and Thomas Bosbach. See another article.
The cipher in a letter of 26 December [1521?] from Emperor Charles V is unsolved. See another article.
The cipher in a letter of 24 September 1527 from Juan Perez, ambassador in Rome, to the Emperor appears to be unsolved. See another article.
BnF fr.3022, f.16, is a report to Charles V in Spanish, undeciphered. See another article.
BnF fr.3022 contains letters of Marquis of Gasto in cipher to Charles V. See another article.
I solved in 2018 a cipher used in a letter (1529) to Emperor Charles V from Suarez de Figueroa, Spanish ambassador in Genoa. See another article.
Before that, I tackled a cipher in a letter (1543) from Suarez de Figueroa to Prince Philip. After some initial success, however, a plaintext was found in the archives. See another article.
(Stub) I learned from George Lasry and from Cipherbrain that a letter in 1547 from Charles V to Jean de Saint-Mauris, his ambassador in Paris, preserved in Bibliothéques de Nancy, was broken by Cécile Pierrot, Pierrick Gaudry, Paul Zimmermann, and Camille Desenclos. Their presentation on 23 November 2022 received a wide media coverage.
Undeciphered letter of 1551 (PARES)
Undeciphered letter of 24 August 1551 (PARES)
For contemporary Spanish ciphers, see another article.
The letter (PARES) is undeciphered. The cipher looks like (but, of course, not the same as) Cg.6 (1571). (For Spanish ciphers at the time, see another article.)
Of many unsolved ciphertexts of which keys have been found (by myself or others), only some are given in this article. I specifically mention BnF es.132 here because it may contain interesting materials for historians. It includes many undeciphered letters from Philip II and Antonio Perez to Juan de Vargas Mexia. The keys were not known at least as of 2012. In 2020, I identified the four keys used in this volume (I cracked one myself). This will allow historians to read the letters in the volume. See another article for details.
The earliest known specimen of ciphertext in France is unsolved. See another article for some unsolved French ciphers in the reign of Francis I. (In 2021, the cipher between Calvimont and Duprat was solved by Norbert Biermann. In 2022, the cipher of Mr. de Gramont, Bishop of Tarbe (1529) was solved by George Lasry. A cipher between Bayonne and Montmorency (1528-1529) was also solved by George Lasry.)
A letter from Jean Hurault[?], French ambassador in Venice, appeared undeciphered (see another article). In 2022, George Lasry solved it (see another article). It showed that an attached plaintext corresponds to this ciphertext.
BnF fr.20974 is a collection of ciphers used in the correspondence of the Duke of Guise (ca.1556). It includes some undeciphered ciphertexts. I solved one cipher (no.8), but two (no.1 and no.7) remained unsolved. See another article.
In March 2021, George Lasry solved some part of the no.7 ciphertext (private communication). In the same month, Norbert Biermann independently solved a large part of it, after which he found that the key is on p.93 of BnF fr.20974 (private communication).
In the same month, Biermann found that no.1 could be read with the Nevers-Piles cipher.
A letter dated 27 April 1567 partly in cipher from Catherine de Medicis to Philibert du Croc is reproduced in Paul Destray (1924), Un diplomate français du XVIe siècle: Philibert du Croc (Gallica) (p.53 and the leaf next to p.56). The clear text preceding the ciphertext ("jay recu du sr x3 lettres en datte") may suggest the ciphertext begins with "du." See another article for contemporary ciphers.
A letter in cipher from Charles IX to Philibert du Croc is reproduced in Paul Destray (1924), Un diplomate français du XVIe siècle: Philibert du Croc (Gallica) (p.80 and the next leaf). Although the letter is not dated, the endorsement indicates Philibert du Croc was then ambassador in Scotland. In 2022, George Lasry solved it (see another article).
Some letters from Lodovico Birago to the Duke of Nevers from 1570-1572 preserved in BnF fr. 3251 can be deciphered by using keys reconstructed from already deciphered materials. These keys, however, do not solve a letter dated 13 November 1571, which is in a different, numerical cipher. (See another article).
A letter of Nicolas Potier de Blancmesnil to the Duke of Nevers, dated "cer dernier juin" in BnF fr.3633, f.24, contains some phrases in cipher. See another article.
BnF fr. 4736 contains (f.87, no.36) a letter from D'anzay to Henry III, dated Copenhagen, 14 October 1574, which contains some undeciphered lines in cipher. See another article for contemporary ciphers.
A letter in cipher from Villeroi to Henry III of France was sold at an auction. It is wholly in cipher except for the complimentary ending: "Sire ie prie dieu conserver vre mté en parfaicte santé / De Bergerac ce viiie jour de sepbre 1577" and the signature "Vre très humble très obéissant & très obligé subjet & serviteur. Deneufville" in Villeroi's hand. (The date is shortly before the Treaty of Bergerac (Wikipedia) was made between the King and the Huguenot princes.)
In 2022, George Lasry solved it (see another article).
I posted undeciphered letters to Mr de Mercoeur probably from the Duke of Guise in my blog (BnF fr.15564, f.27, f.78, f.119, f.142). In 2022, George Lasry solved it (see in another article). He also found there is a short ciphertext in f.151 (which I failed to mention in the blog, but mentioned in another article, which also presents several other undeciphered ciphertexts, including the one below). He says it does not decipher with the same key and is probably too short for cryptanalysis.
Henry IV's letter to Savary de Breves dated 5 January 1610 remains unsolved. See another article.
In February-March 2021, George Lasry solved a significant part of this ciphertext (private communication).
Short passages in cipher in letters from Regent Marie de Medici to Mr. de Breves appear to be unsolved. See another article.
I solved in 2019 a cipher used in a letter addressed to Cardinal Schiner (1520), only to find that it had already been solved by Grégoire Nicollier, Matthieu Jacquemet, and Gilles Evéquoz. The cipher is a simple numerical cipher, but it is interesting in being an early specimen. See another article.
Lasry et al., "Deciphering papal ciphers from the 16th to the 18th Century" (published online in 2020 at Cryptologia) systematically studied many ciphertexts found in the Vatican archives and recovered no less than 16 keys out of 21 used. (Reportedly, it was done between 24 April and 9 June 2019 (p.492). The ciphertexts presented in the "Vatican Challenge" mentioned below were ones collected as part of this project.) The paper discusses algorithms for clustering ciphertexts into groups of ciphertexts enciphered with the same key; solving fixed-length homophonic ciphers; solving variable-length homophonic ciphers; and solving polyphonic ciphers. By the systematic study of many materials, the paper showed the sophistication of the papal ciphers in the 16th century was lost in the 17th century, when only simpler, less secure ciphers were used.
A private email made me aware of three new Vatican challenge ciphertexts posted by George Lasry in 2019 at MTC3:
The Vatican Challenge - Part 3 ... a short message dated "Brusseles 9 Oct. 1721", consisting of one- to four-digit figures. This was solved in George Lasry and Paolo Bonavoglia, "Deciphering a Short Papal Cipher from 1721" (HistoCrypt 2022).
The Vatican Challenge - Part 4 ... a letter from the bishop of Senigallia to the Secretariat, consisting of figures. The cleartext seems to include the date "Marzo 1536." This was solved in August 2019 by Thomas Bosbach, who found a matching plaintext in a book (Lasry et al., "Deciphering papal ciphers from the 16th to the 18th Century" (Cryptologia)).
The Vatican Challenge - Part 5 ... The cleartext seems to include the date "Aprile 1542." According to the introduction of the challenge, this letter is in a bundle of "Lett. Orig. e cifre del card. Farnese al nunzio, 4 oct 1539-24 nov. 1548. ff. 7-123."
See my blog for some observations.
Two ciphers of papal nucios (1625, 1628) were posted as challenge problems at MTC3 in 2018 (Vatican Challenge Part 1, Part 2). Thomas Bosbach and Norbert Biermann solved them independently. I was the third to solve them. See another article.
Before that, I solved three Vatican ciphers from 1593. See another article in Cryptologia.
Solution of a similar ciphertext (1573) was described by Albert C. Leighton back in 1969. See another article
Many of the ciphers presented in works of Giovan Battista Bellaso in the mid-16th century were solved by Tony Gaffney in 2009. The remaining four were solved by Norbert Biermann in 2016-2018. See the links at the end of another article (in Japanese).
BnF fr.4715 in the French archives contains many undeciphered letters. I could solve some (at least partially) but three remain unsolved despite their seeming simplicity. (One (no.61=f.84)) was solved by George Lasry in 2020. In 2022, George Lasry solved another (no.59=f.82) and achieved interim results for the other (no.62=f.85).) See another article.
BnF fr.4712 contains some undeciphered letters. See another article.
The French archives has many other cipher materials. In some cases, they can be solved with keys reconstructed from letters with decipherment. See the following articles:
Reading an Undeciphered Letter of the Duke of Mantua (1590, 1593)
Reading Undeciphered Letters to the Duke of Savoy (1593)
Reading an Undeciphered French Letter from Antwerp (1580)
Cinq Cents de Colbert 33, a volume containing despatches deciphered by Viete, contain some undeciphered original letters apparently left undeciphered by Viete. See another article.
(f.539) Cardinal de Joyeuse to Villars, admiral de France, Rome, 15 February 1594
(f.555) Senecey to Archbishop of Lyon (This was solved by George Lasry in 2020.)
Ms. 994 of National Library, Madrid, contains an undeciphered letter from Venetian secretary Marco Otthobon [Marco Ottobon] to ambassador, Juan Mocenigo [Giovanni Mocenigo], dated 27 April 1589. Valle de la Cerda appears to have solved it, but his solution is lost. See another article. The cipher symbols consist of an alphabetical letter followed by Arabic figures. For this kind of ciphers, see another article.
BnF Clair 369 (Gallica) contains a partially enciphered letter dated Rome, 13? November 1616 from Cocquet to Mangot (f.316). See another article for some contemporary ciphers.
Ms. 994 of National Library, Madrid, contains ciphertext solved by Luis Valle de la Cerda. The cipher was in a scheme invented by a Milanese, Jerónimo Sertori. Valle not only broke it but indicated that he had devised the same scheme fifteen years earlier. However, his solution is not extant. See another article. (According to Nick Pelling's blog ("Girolamo Sirtori Cipher Mystery"), Eloy Caballero got close to the solution in 2012.)
A letter of fra Guglielmo Vizani, 8 October 1637, has portions that appear to be in an unsolved cipher. See another article.
A simpe substitution cipher used by Albrecht von Wallenstein during the Thirty Years' War was solved by Nagra in 2016. See another article.
Emperor Ferdinand III and Archduke Leopold used cipher in their private correspondence in 1640-1645. In 2018, Thomas Ernst succeeded in codebreaking. The Habsburg brothers' cipher turned out to be essentially a numerical cipher with some figures guised in graphical symbols. See another article.
Two homophonic substitution ciphers of Imperial ministers from the time of the Thirty Years' War were solved in 2018. One was solved by Thomas Bosbach and Nagra independently. The other was solved by Thomas Bosbach. See another article.
A letter (1646) of Carl von Rabenhaupt (Wikipedia), a Bohemian who fought for the Protestants during the Thirty Years' War, to Amalie Elisabeth, regent of Hesse-Kassel (Wikipedia), is presented in Klausis Krypto Kolumne (2016) and Crypto-World (7-8/2013; 9-10/2013; 11-12/2013). The cipher consists of two- or three-digit figures and other symbols.
This was broken in 2020 by finding the key in the archives. See another article.
Some undeciphered ciphertexts of Melchior de Sabran, a diplomat then resident in Genoa (1630-1637), are preserved.
I solved one cipher used between Abel Servien and Sabran in 1632, thanks to two independently enciphered copies with some words in the clear (see another article).
The cipher used in Louis XIII's letter, dated Dijon, 28 March 1631, to Sabran was solved by George Lasry in 2022 (see another article). It was also used in short passages in cipher in other letters from Sabran to Bouthillier from 1633-1635 (See another article).
The cipher used in what seems to be an enclosure of Odoardo Farnese's letter, dated Plaisance, 27 May 1637, to Sabran was also solved by George Lasry in 2022.
Short passages in cipher in a letter to "Mr de ch.gr" appears to be in a different cipher, yet unsolved.
Cardinal Richelieu's letter from 1641 mostly in cipher was solved by Norbert Biermann in 2020 (Klausis Krypto Kolumne). See another article for some background.
An unsolved letter from Henri Brasset, a French resident in The Hague, to "Vostre Eminence" (Mazarin?) is in BnF Clair 421. The figures with diacritics probably represent syllables alphabetically arranged and some words and names, as with other ciphers at the time (see another article). You can post comments or solution in my blog, where I posted my provisional transcription and initial comments.
Melanges de Colbert in BnF contains some undeciphered letters. Some can be read with a key reconstructed from other materials, but some still remain unsolved.
Melanges de Colbert 172, f.23, is a letter from Louis Béthune, Duke of Charost (1673). A short paragraph is in an unidentified cipher.
Melanges de Colbert 168bis, f.553, is a letter from the abbé de Gravel to Comte de Maulevrier (1674), and has short passages in cipher.
Melanges de Colbert 127, f.349, has a few words in cipher (1665).
See another article.
Calendar of the manuscripts of the Marquess of Ormonde, K.P., preserved at Kilkenny (Internet Archive), p.28, includes two letters partially in code from Lord Maltraverse (Wikipedia), MP in the Parliament of Ireland, to Ormonde in 1634-1635.
The underlined occurrences "44 79" and "79 44" seem to indicate these are combinations of frequent letters. Many cipher sequences are delimited with 91-111, which may be nulls.
Unsolved letters from Charles I to Boswell (2 November 1643) and Nicholas to Boswell (2 November 1643) are preserved in The National Archives (TNA). (The recipient may be Bosvil, Bothwell, or the like.) My provisional transcription is available on my blog.
An undeciphered letter dated 13 May 1644 is found among papers of Sir Richard Forster, treasurer of Queen Henrietta-Maria's household, according to Karen Britland (2013), "Reading between the lines: royalist letters and encryption in the English civil wars", Critical Quarterly, vol.55 (4), pp.15-26. It is in the hand of Forster, and may have been addressed to Henrietta-Maria. The Queen had, leaving Oxford threatened by the parliamentarians, come to Exeter, where she would give birth to a girl in June before leaving for France. See my blog for a transcription.
A letter in cipher printed by Davys (1737) has been solved by the present author in 2011. The letter was one long known to historians. See another article.
While Charles I used various ciphers, a passage (8 April 1645) in the private cipher for use with the Queen appears to remain undeciphered. See another article.
Charles I's four letters in cipher during his captivity in the Isle of Wight (1648) remain undeciphered. See another article.
In 2021, the cipher used in two of the four was solved by Norbert Biermann and Matthew Brown (see another article).
An encoded letter Prince Rupert received from his brother soon after the Battle of Naseby is printed in Memoirs of Prince Rupert, and the cavaliers by Eliot Warburton (Google), p.133 and appears to be unsolved.
The highest number is 398. Considering that two-digit figures do not appear in succession, the code seems to be different from the ones Rupert used with Charles I or Nicholas at about the same time (see another article).
The British National Archives has an educational page that shows an image of a letter in cipher from Prince Maurice to Lord Digby dated 31 August 1645. Although it is not "unsolved" (see another article), it may be included here because the page encourages the reader to crack the code because "We don't know!" the content.
Some passages in cipher in Charles II's letter to the Duke of Hamilton (1650) remain undeciphered. The key may be found in the collection of letters deciphered by John Wallis, deposited in the Bodleian Library, which includes royalist letters from this year. See another article.
The cipher of Charles II's letter by a pseudonym J. Westrope was broken by Eric Sams and Julian Moore in 1977. The letter was one long known to historians. See another article.
Some letters by Hyde in Brussels just before the Restoration have superscription in cipher (see another article).
Examples of superscription in clear in the same volume are "For Mr. Brookes" (p.313 in Latin edition), "For Mr. B" (p.335), "For Mr. Burges, these" (p.339), "For the Lord General Monk" (p.441), "For General Monk" (p.442). The undeciphered superscription will also indicate recipients. 729 may be "for."
An enciphered passage in the memoirs of Philip Stanhope, 2nd Earl of Chesterfield, was kindly brought to my attention by Richard Merriman in 2022. Geoge Lasry solved it in an instant. See my blog.
Thurloe State Papers include some undeciphered Dutch letters, for example: Beverning and Vande Perre to the Dutch ambassador Boreel at Paris, 1 September 1653 (NS). See another article.
Thurloe State Papers include some undeciphered intercepted letters, for example:
An intercepted letter of du Gard in a letter to White, Brussels, 10 June 1656 NS (British History Online)
An intercepted letter, Brussels, 12 August 1656 NS (British History Online)
An intercepted letter, from Jo. Waddall, 22 August 1656 (British History Online)
Louis XIV's instructions in code to Castaignere, French ambassador in Constantinople, dated July-August 1690 are printed in Paul Rycaut, History of the Turks (Google), p.453 ff. This appears to be the letters John Davys deciphered around 1701 (see another article) but his deciphering is not known. The present author partially solved it (see another article).
Louis XIV's instructions in code to the Duke of Chaulnes, French ambassador in Rome, dated 10 July 1690, remain unsolved. See another article. While I was writing the relevant section therein, discussion started in Klausis Krypto Kolumne.
Harry Thompson, The Man in the Iron Mask prints in the appendix a letter in a French cipher that John Wallis could not solve (see another article). It is one of the letters from Louvois to Lauzun in Ireland (27 May 1690), written about a month before the Battle of the Boyne.
Norbert Biermann solved this by finding the key in the archives in 2020.
A coded letter of 1691 addressed to General Catinat appears to remain unsolved (see another article).
A coded letter of 15 September 1702 written by Marshal Catinat remains unsolved (see another article).
An encoded letter from Admiral D'Estaing to Gerard, French minister in Philadelphia, dated 30 April 1779 is preserved in William L. Clements Library, Clinton Papers, "vol 64:14". It begins as follows:
The present author identified four French diplomatic codes in the same period (see another article) but none of them seems to decode this. The highest number used in this letter is 597, suggesting a smaller code than the diplomatic codes.
A decoded, but not identified code of Luzerne, French minister to the United States, is presented on a blogpost.
A short ciphertext in a letter from George Stepney to the Earl of Manchester, dated Vienna, 23 March 1702, preserved in the Manchester Papers, is not deciphered. (Some other undeciphered letters in the Manchester Papers can be read with the key (THE=454) preserved with them.)
The breaking of a cryptogram left by the Swiss mathematician Leonhard Euler by Pierre Speziali (1953) and Hans Rohrbach (1973) are described in an article in Japanese.
There is a portion in cipher in a letter James Madison received from an Italian, Philip Mazzei. See another article.
In 2015, when this page was mentioned in a German cryptology blog, Armin Krauss immediately responded with his solution. See another article.
Patterson devised a new cipher system and sent Thomas Jefferson a challenge, which was deciphered in 2009 by Lawren M. Smithline. See another article.
An Encoded letter to Marshal Marmont in 1807 is reproduced in J. Vilcoq, "Le Chiffre sous le Premier Empire", Revue Historique de L'Armée No.4 (1969). It begins with "Vous avez du recevoir Monsieur le General Marmont mes lettres des 8.14 et 20 courant" and the rest is wholly in code. The code consists of two-digit figures as well as alphabetical letters and other symbols.
Considering that Marmont used a relatively simple code of 150 entries in 1811 (see another article), this would not be a very complex system.
An Encoded letter from Berthier to Napoleon dated 22 December 1812 is reproduced in J. Vilcoq, "Le Chiffre sous le Premier Empire", Revue Historique de L'Armée No.4 (1969). It begins with the following. See another article for Napoleonic ciphers that may be similar to this code. Considering that it bears a note: "Duplicata, Chiffre du Prince de Neufchâtel, La Primata a été déchiffrée", its decipherment may be found in some archives.
A letter of 30 August 1808 from John Armstrong, US minister in France, to James Madison, secretary of state, has an undecoded postscript. The known plaintext of other letters may give a clue to its solution. See another article.
In 1841, Edgar Allan Poe wrote a series of articles in a magazine, in which he solved simple ciphers sent by the reader. At the end of the series, Poe published two cryptograms sent by W. B. Tyler, which he did not solve. The first was solved independently by Terence Whalen (dissertation in 1991; published in 1994) and John Hodgson (1993), while the second was solved by Gil Broza in 2000. See another article.
In 2016, the project "Decoding the Civil War" started to transcribe and decipher about 16,000 telegrams from The Thomas T. Eckert Papers. These papers belonged to Thomas T. Eckert, the head of the Union military telegraph office, and, though long thought to have been lost, were recently acquired by The Huntington Library. They include Union correspondence that has never been published. See another article for an overview of Civil War codes and ciphers.
Indiana Memory Digital Collections has cipher telegrams to Brigadier General Robert H. Milroy. One is from J.D. Cox, dated 3 December 1862 (here), and another is from G.M. Bascom, dated 4 December 1861 (here).
Probably, these employ a simple route transposition system. The context would be found in OR vol.24, around p.830 (see another article).
An encoded letter from J.E. Johnston to Robert E. Lee, dated 8 April 1862, is found at Civil War Day by Day. The image shows the recipient could not decode the message. Davidsch found the right dictionary, which led to the following reading (see another article).
An encoded letter from Lieutenant Barney, commanding CSS Harriet Lane, to Mallory, Secretary of State, remains unsolved. The dictionary is said to be a Webster (see another article).
The highest page number in the message is 262 and the highest line (or entry) number is 85. The column is either 1, 2, or 3. While there are many dictionaries bearing the name of Webster (The Online Books Page), 3 columns and 85 lines (assuming the latter indicates an unmanipulated line number directly) almost limits the candidates to the unabridged with its most condensed format (for example, Webster's Handy Dictionary (1877) has three columns but only 64 lines). One may rather rely on the relative location of the coded words in the dictionary (see another article for illustration of such a method), with the help of the context found in OR Navy or other sources.
The Canadian government used Slater's code (Telegraphic Code (1870) by Robert Slater). In 2021, Matthew Brown found more than a hundred encoded telegrams from John A. Macdonald, the first prime minister of Canada or people around him and succeeded in decoding most of them. Since Slater's code involves translating a word into a number and, after some manipulation of the number, translating it back into a different word, knowledge of the codebook is not sufficient to allow reading the telegrams in code. See another article for his codebreaking technique and the results.
An encoded telegram from the British consulate in Lüderitz (Wikipedia) (then German South-West Africa (Wikipedia)) to the Foreign Office in London is posted in Klausis Krypto Kolumne. It consists of 43 five-figure groups: "68195 71235 ...."
An undecoded telegram to Sun Yat-sen (1916) is preserved in the Japanese archives. For the most part, it consists of ten-letter groups. It seems certain that the message was first encoded with a conventional Chinese telegraph codebook, and then two-digit figures were translated to two letters (consonant+vowel) with a code condenser.
Another article presents two contemporary code condensers used by Junzaburo Yamada, who supported Sun Yat-sen, and also provides my decoding of Sun Yat-sen's telegrams, which were superenciphered by addition of 111 instead of using a code condenser.
The Japanese found out the superencipherment, but it is not known whether they identified the code condenser. A clue may be found from the fact that the typical Chinese codebook at the time did not have the figures in the 9000s.
See my blog for my unsuccessful attempts with the known code condensers.
The Japanese archives preserves a telegram from Huang Xing to Lin Hu and Li Genyuan (1916). They were in exile in Japan, and opposing Yuan Shikai separately from Sun Yat-sen.
The syllable structure is different from Sun Yat-sen's code condenser above. Actually, the syllables (no Cs or Ls, SHI, CHI instead of SI, TI) conform to the Japanese syllabary. There remains a possibility that the enciphering is something different from a code condenser.
See my blog post.
It may be inaccurate to call this "unsolved" because it was probably solved by Yardley's Cipher Bureau at the time. It is printed in Yardley's The American Black Chamber (1931) p.251 as a typical example of Japanese displomatic code messages (though he does not say this was the first which he solved and demonstrated in the book). The sender appears to be Uchida Kosai, foreign minister from 29 September 1918 to 2 September 1923. See another article for various codes deciphered by Yardley and another article for details of one specific code dubbed Jp.
In 2008, the Zhongshan Warship Museum called for solution of encrypted telegrams found in SS Zhongshan, which was sunk by the Japanese bombing in 1938. As of 2009, 352 out of 891 were solved. I have not located the primary sources of these telegrams. See another article.
The German Enigma cipher during World War II was broken by the Allies but three unsolved messages intercpeted in 1942 were found. Two of them were solved by distributed computing of Stefan Krah's M4 Message Breaking Project in 2006 and the last was solved by Dan Girard in 2013. CNET carries Graeme Wearden's report in 2009.
A photo(!) of an unsolved Enigma message sent on 10 January 1945 by a deputy of "Oberbefehlshaber Oberrhein" (Supreme Commander of Upper Rhine Area) (to which post Heinrich Himmler had been appointed in December 1944) is presented in Klausis Krypto Kolumne. This article also cites further works in recent years to break original Enigma messages from World War II. Another article of the blog reports reconstruction of Bombe.
A serial killer known as Zodiac left four ciphertexts: Z408, Z340, Z13, and Z32 (so called becau se of the number of characters contained in each cryptogram). The first one, Z408, was broken in a week. Z13 and Z32 are too short to be broken without any side channel information. Z340 has been attracting world-wide attention, but remained unsolved. In 2020, it was broken by David Oranchak, Sam Blake, and Jarl Van Eycke. See another article in Japanese.