Can a Vigenere Cipher be Broken without Key Repetition?

I've been interested in whether a Vigenere cipher without key repetition can be solved. If the key is random, it is a one-time pad and is theoretically unbreakable. On the other hand, if the key is a natural language text, I heard such a cipher (called a running key cipher) can be solved. But how?
I uploaded a new article about this: "Solving Running Key Ciphers (Manually/Digitally)".

A Florentine Polyalphabetic Cipher from the 1480s

I learned in a paper by Marco Vito that the instruction for a Florentine cipher from the 1480s prescribes its use as a polyalphabetic cipher. This is the only actual example of a polyalphabetic cipher before 1560 (not counting the treatises by Alberti and Trithemius as actual examples).
This induced me to collect specimens of polyalphabetic ciphers known to me as a new article, "Polyalphabetic Ciphers before 1600".

Japanese Cipher Disks during the Meiji Period

I uploaded a new article in Japanese about cipher disks used in Japan during the Meiij Period. At the time, Japan was just starting to use Western-style ciphers and the cipher disks were not used for a polyalphabetic cipher, but only for switching the monoalphabetic substitution table.
While some of these have already been mentioned in some of my articles, I sorted those with new materials I came across recently, including (1) telegrams in relation to the assassination attempt on the Tsarevich (1891) (Wikipedia) and (2) photos of actual cipher disks used in post offices.

Real Messages Encrypted by Japanese RED Machine

Two authentic messages encrypted by using RED (1937) are recorded in a document declassified by NSA (REF ID: A71152), which I recently learned from Jean-François Bouchaudy.
The messages are:
(1) Ambassador in Berlin ("Mushakoji" [Wikipedia]) to Foreign Minister in Tokyo (Arita [Wikipedia]), 4 January 1937 (ciphertext, raw decryption, English translation).
(2) From Tokyo to ambassador in Berlin, 8 January 1937 (ciphertext, raw decryption) with decryption in romaji and English translation (somehow dated "9 January 1937").

Bouchaudy learned of this document from George Lasry in June 2025, who quickly found the key to the two messages. Then, Bouchaudy succeeded in reproducing the decryption by the RED simulator he developed.
He reports the results on his website, in which I contributed in parsing the raw decryption of the first message into words in Japanese.

For people interested in ciphertext encrypted by RED, Bouchaudy "created" 9 problems in the form of historical correspondence among US codebreakers. He tells us they (not authentic RED messages) are based on the wiring of the examples given in Alan G. Konheim (2007), Computer Security and Cryptography (which deals with RED and PURPLE in Chapter 7).

Deavours and Kruth (1985), Machine Cryptography and Modern Cryptanalysis has a paragraph in RED (p.213), but Bouchaudy tells us it is different from RED because it "has no Period".

"Cipher Disk" before Alberti

I recently learned that before Leon Battista Alberti's treatise, there had been ideas similar to his cipher disk. In particular, Giovanni Fontana's disk for mnemonics looks very similar to Alberti's cipher disk, though there is no evidence Alberti knew this. (Another interesting thing about Fontana is that his ideas are described in a manuscript booklet almost entirely in cipher.)
I uploaded a short article about this: "Giovanni Fontana's "Cipher Disk" (ca. 1430) in an Encrypted Booklet". (This was delayed several weeks because of troubles of my PC....)

An Original French Naval Code (1778)

An original French code (1778) is reproduced in Pierrot, Chaline, Damoiseau-Malraux, Mekhail, Perret (2025), "A Caribbean Directory-based Encryption during the American War of Independence" (HistoCrypt 2025).
It was for D'Agoût, governor of Saint Domingue (1777-1780). It is a two-part code consisting of a table for encoding in alphabetical order and a separate table for decoding in numerical order. The substitution table assigns three figures for the vowels and one figure to the other letters and the ampersand.
The nomenclature includes entries for syllables and common words. As was typical with French ciphers, some variants are covered by one entry: "action, s" (for "action", "actions"), "it, e" (for "it", "ite"), "puis, que" (for "puis", "puisque").
It should be noted that the codes are made on printed templates. The template for encoding has syllables and words printed in alphabetical order, with a section "NOMS de Lieux" in which place names are not printed and are filled when preparing a code for a specific application. The decoding table has printed numbers 1-850.

Pierrot et al. (2025) reports their reconstruction of a similar code used in 1782 between Guillaume de Bellecombe, governor of Saint Domingue a few years after D'Agoût (1782-1883), and the Secretary of State for the Navy. The number ranges up to 857. There might have been a necessity for additional entries 851-857 beyond the template range 1-850.

Contemporary French diplomatic codes I know had 1200 entries (Barbé-Marbois (1782), Luzerne (1781), Napoleonic age), larger than these naval codes. On the other hand, the code used in one undecoded message between Admiral D'Estaing and Gerard, French minister in Philadelphia, (1779) ("Unsolved Historical Ciphers") has a highest number 597.

Tuple Ciphers (Enciphering a letter with two letters)

I read about reconstruction of an interesting cipher (1537-1547) in a paper by Lasry, Simonetta, and Biermann in HistoCrypt 2025. It enciphers a letter with two letters, while enciphering a syllable with a letter. Moreover, frequent omission of the dot to distinguish syllables makes it look polyphonic. My new short article "Tuple Cipher -- Enciphering a Letter with Multiple Letters" focuses on the aspect of enciphering a letter with multiple symbols.

Dutch Codebreaking Activities during WWI

Florentijn van Kampen (2025), 'Dutch Cryptanalysis of Four American Diplomatic Codes in World War I' (HistoCrypt 2025) is interesting in describing Dutch codebreaking activities during WWI. Beside that, it presents examples of how code (as opposed to a cipher) can be broken. It is also valuable for me to show sources about the US State Department Codes. I updated "How to Break a Code (Not a Cipher)" and "Red and Blue: First Official Codebooks Adapted for Telegraphy" by mentioning these.

Verification of Decryption of Beale Cipher No.2

I have not paid much attention to the famous cryptogram known as the Beal Ciphers (No.1 - No.3). Many consider them a hoax, but at least the Beale Cipher No.2 can be deciphered by using the Declaration of Independence as a key. I recently read that it has been pointed out that deciphering the Beale Cipher No.2 with the given key is not so straightforward. Indeed, there are numerous errors in the numbers. Although the correct reading might have been achieved by close study, it seems unnatural that such painstaking effort is not mentioned in the original pamphlet.
I uploaded a new article ("Errors in Beale Cipher No.2") about this.
(It's a pure coincidence that it's the Independence Day today.)

A Cipher of Eleanor of Navarre (1476)

I uploaded a short article "A Cipher between Eleanor of Navarre and her Father, John II of Aragon (1476)" presenting a simple cipher used between Eleanor of Navarre and her father, John II of Aragon (1476).

Imperial Ambassadors in the 1520s (Juan Manuel, Alonso Sanchez)

I added entries for Juan Manuel (imperial ambassador in Rome) and Alonso Sanchez (imperial ambassador in Venice) in "Unsolved Historical Ciphers". Searching archives may be more helpful than cryptographic methods for these ciphers with many nomenclature entries.

Lope de Soria's Cipher Letters (1523)

Once I was particularly interested in ciphers of Lope de Soria, a Spanish ambassador in Genoa in the 1520s (see "Tracing the Origin of Vowel Indicators in Spanish Ciphers"). Now some of his cipher letters (1523) are in the DECODE database. They use two keys, printed in Galende Diaz (1992). I added reference to these letters in "Ciphers during the Reign of Emperor Charles V".

(The two recent additions to DECODE, R9531, R9530, are the cipher letters from which I reconstructed the keys in "Spanish Ciphers before Accession of King Ferdinand: 1470-1479" (Academia.edu).)

Oral Cipher in Yoshiwara Pleasure Quarters in Edo

Women in the Yoshiwara pleasure quarters in Edo in the late eighteenth century used a cipher, called the Fukagawa Speak, in their conversation. I learned this from a contemporary novel depicted in a scene in a historical drama I watched on TV the other day. The scheme is simple enough: meaningless syllables "ka", "ke", "ki", "ko", "ku" are inserted in the message. But when used in conversation, the resulting sound would have been unintelligible to untrained ears.
Similar schemes, typically known as the tanuki cipher, have also been used in riddles for kids. (Tanuki means a raccoon dog, but may also be interpreted as "drop ta".)

And, of course, it's not limited to Japan. Even John Wilkins' Mercury (1641) mentions an example of a similar scheme: Ougour plogot igis digiscogovegereged.
A Japanese translation of a mystery novel in English uses the Fukagawa Speak to render secret conversation in "King Tut English" among kids: A-bub-shush-o-lul-u-tut-e-lul-ylum.

I described these in a new article in Japanese, 深川言葉　―　大河ドラマ「べらぼう」に出てきた遊里の暗号.

Substitution Cipher for Hangul Text in a Book for Kids

The other day, I said I was looking for a cipher in Hangul. Now, I've found the book I have been looking for. It is a Korean translation of a book in Japanese, which features a cipher. Although the ciphertext is not in Hangul but in graphic symbols, this is the first ciphertext for Korean I encountered. I uploaded a new article, "Substitution Cipher for Hangul", which supersedes my post the other day.
(There is also a Chinese version of the book. It would be interesting to see how the cipher was adapted into Chinese. The Taiwan version skips the relevant volume (vol.11), so we need to look for Hong Kong or Mainland Chinese version.)

D'Estrades' Ciphers with Mazarin and Others

I added ciphers used in the correspondence of Comte D'Estrades in "Ciphers Early in the Reign of Louis XIV". Most of the keys are now identified, but a key for a letter in Italian (BnF Clair. 577, p.629) is not. The interlinear decipherment of the first few words (wihch I partly parse as `41(L) ¨33(AR) ¨36(MA) ′56(TA) ¨56(NA) ′61(VA) `42(LO)) may allow one to reconstruct the whole code.

(15 Mary 2025: I uploaded the key for the remaining Italian cipher reconstructed by George Lasry and Norbert Biermann. My parsing should be corrected in various ways: `41(LA) ¨33(R) ¨36(MA) ′56(TA) ¨41(NA) ′61(VA) `42(LE).)

Chinese Telegraphic Codebook for Phrases (1948)

I got a copy of a Chinese telegraphic codebook (1948) including not only single characters but also phrases. It is Cheng yu Dian ma (成語電碼) already described in 「電碼――中国の文字コード」 (an abridged English version), but I now added three photos.

 

A Cipher in Italian used between Cardinal Gualterio and the French Foreign Minister Torcy (1718)

I added a section "Gualterio-Torcy Cipher (1718)" for a cipher used in a letter from Cardinal Gualterio to the French Foreign Minister Torcy in "A Syllabic Cipher (ca.1715) of Cardinal Gualterio Reconstructed Manually".

Yardley's Diplomatic Novel, Red Sun of Nippon (1934)

I enjoyed reading Yardley's diplomatic novel, Red Sun of Nippon (1934). It is a story of a young American diplomat and his half-Chinese, half-white girlfriend, set in the contemporary Washington D.C. It develops around an important document which would be evidence of the Japanese intention of establishing a buffer state in Manchuria. As expected from Yardley, breaking of Japanese diplomatic code plays some role. See my new article, "Yardley's Diplomatic Novel, Red Sun of Nippon (1934)".

Substitution Cipher in Hangul

Japanese text can be written with some fifty kana, which can be enciphered with substitution ciphers. (Historical examples can be seen e.g., at 明治日本の暗号いろいろ). The Chinese language employs thousands of characters, so any practical encryption in Chinese is based on numerical code of those Chinese characters.

What about Korean? I have long wondered whether substitution cipher is possible in Korean script (Hangul). This is because Hangul text is not simply a series of Hangul letters. Instead, two or three Hangul letters (consonant+vowel or consonant+vowel+consonant) are combined to form a syllabic block, which is the unit of writing (morphemic block). Moreover, placement of letters within a block differs depending on the vowel. The vowels for A, E, I (which have a vertical axis) are written to the right of the initial consonant, but the vowels for O and U (which have a horizontal axis) are written under the initial consonant. So, simply substituting one Hungul letter for another may result in a letter sequence that cannot be formed into morphemic blocks.

Now, I learned possible simple solutions to my question. There may be two ways for substitution ciphers to work in Hangul.
(i) Substitution is applied in separate groups for consonants and vowels. This way, the "consonant-vowel" grouping is preserved in substitution. This is the approach that AI (Copilot) on my new PC gave to my question. Actually, AI at first gave an example of a Caesar cipher of consonants alone. When I asked about vowels, AI gave a separate substitution table for vowels. I do not know whether this simple scheme really works for actual Hangul text. (For example, I hear not every consonant can be the third element in a block; there are letters that are not simple vowels or consonants.) To my repeated requests to give actual examples, AI simply ignored "in Hangul" and gave websites for the Caesar cipher in Japanese!

(ii) Substitution is applied to morphemic blocks rather than to letters. Unicode registers 11,172 Hangul morphemic blocks (399 consisting of two leters + 10,773 consisting of three letters). If we think this whole set as a single alphabet, substitution cipher can be used. (An attempt to use the Vigenere cipher in Hangul (pdf in Indonesian?) seems to use this approach.)

Googling now finds applications to use cipher in Hangul, but I still cannot find actual examples of substitution ciphers in Hangul. Considering that Hangul was not used in official documents until 1894 and that I have seen use of numerical codes for telegraphy (posted here), substitution in Hangul may not have been common historically. 

 

Yardley's Codebreaking of a Chinese Book Cipher (1939)

Herbert O. Yardley wrote The Chinese Black Chamber after Word War II, posthumously published in 1983. It describes his days in China from 1938 to 1940, where he was invited by the Chiang Kai-shek's government to work on Japanese codes and ciphers. Codebreaking of one particular cipher is described in some detail, which I described in a new article, "Yardley's Codebreaking of Book Cipher used by Pro-Japanese Traitors in China (1939)".