Cryptiana Discussion Forum

Some months ago, I wrote about a British book cipher during the American Revolutionary War.

Two undecoded telegrams to Union General Milroy were solved by Richard Bean, who had Claude Opus 5 solve them.

I updated my recent article "Korean Cryptography" with additional specimens from John Milmore (2002), #1 Code Break Boy.

How long did it take for a secretary to decipher a despatch?

Mirabeau's undecoded letter (1787) (see my blogpost) reminded me of the famous cipher devised by him, which is now described in a new article, "Mirabeau's Cipher".

In Western history, there are many prominent historical figures who used code/cipher.

I have been wondering how Hangul script can be enciphered.

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).

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).

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.

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

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

More than half of the symbols are nulls in a letter (1586) from Henry of Navarre, later Henry IV of France.

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".

In the age of telegraphy, commercial codebooks were occasionally adopted by governments.

What happens to secret codes when a nation comes under control of another?

Of the Zodiac ciphers, Z13 (thirteen characters long) is considered too short for cryptanalysis.

Martin de Acuña, an agent for Philip II, used a pigpen cipher in his letters to Philip II.

I added a mention of a letter from Hugo de Moncada, Viceroy of Naples to Emperor Charles V, dated 6 October 1524 (BNE, see Carta 9) in "Scholarly Studies on Ciphers in the Reign of Emperor Charles V".

Further to the call for help to decode a letter from John Armstrong (20 February 1808) for Madison Papers in a post last year, I made two updates in "An Outlier Code in Armstrong-Madison Correspondence (1808)" (marked "June 2026"). (1) The editors of Madi…

I noticed many ciphertexts from Emperor Charles V's time were recently uploaded in the DECODE database.

It was only two months ago that I presented a French code based on materials from Alexandre Pillon.

Three historians have achieved complete transcription and decipherment (as well as full translation into English) of a partially enciphered letter from Pedro de Ayala to Ferdinand and Isabella dated 25 July 1498.

Barbeu-Dubourg is known for translating Benjamin Franklin's work into French.

During the American Revolutionary war, a book cipher was used between Frederick Haldimand, Governor in Quebec, and Sir Henry Clinton, Commander-in-Chief in New York.