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.
The reader is kindly asked to provide information if he/she knows or attains decipherment of any of these.
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 remains unsolved despite their seeming simplicity. 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)
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.
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 ciphertext has not been deciphered yet.
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.
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 letters in cipher during his captivity in the Isle of Wight (1648) remain undeciphered. 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. 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.
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) (British History Online).
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).
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.
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.
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.
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 ...."
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.
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.