I’m excited to report that the challenge set in my blog post of 4 August 2025 has been met. The letter sent by William Perwich on 9 April 1670 from the court of Louis XIV in France has been successfully decrypted, not once but twice.
The collection
Read Ruth's first blog post about the mysterious letter.
On a murky Monday in late September, I started work to find my inbox brightened by two emails with solutions which made sense, both in the context of a columnar transposition of the text and because they included gossip at the court also reported by the Venetian ambassador on the same day.
The cryptographers who cracked the cipher were Matthew Brown (working alone) and Dr George Lasry, Professor Norbert Biermann and Tomokiyo Satoshi (working together). The latter team are well known for deciphering a group of letters from Mary Queen of Scots, found in the National Library of France, in 2023.
The tricky aspects to the cipher were working out how many columns it was in (20) before randomly rearranging them until they formed recognisable words, and identifying all the letters which were ‘nulls’, i.e. should be discarded before attempting to decrypt (a number it is impossible to be certain about without Perwich’s original cipher key).
The letter frequencies pushed the cryptographers in the right direction. Most letters appeared a usual number of times for the English language but, exceptionally, there were 8 Qs. On noting that 6 of these were towards the right margin, they were made aware that each line was probably completed with nulls.
Letter from William Perwich to Lord Arlington, 9 April 1670 (New Style). Catalogue reference: SP 78/129, f.180r
The solution was reached by using codebreaking software the team had developed along with extensive manual work, in part required because Perwich had mistakenly omitted a couple of letters in his ciphertext.
Solution grid. Deciphered grid by Biermann, Lasry and Tomokiyo (October 2025)
After finessing the results, the final decryption reads:
The souldiers grumble much that the king is of late growne cool towards them and gives them not the encouragement in their addresses as hee used. They complain hee is wholly given up to his mistresses who are no enemies to 97 nor peace and consequently dissuade the 60 96 pursuing the French manufactere with vigour knows that peace can onely advance his designs I heard a great man say that the 61 sayd to his brother he wisht him not to oppose madam's going for 40 becaus her journey was for the interest of his kingdome wherupon most do boast of an alliance [likelyhood]
You will note that some numerical codes, standing in for names and places, remain in the deciphered letter. These were a separate system, impossible to crack without the key, especially as there were two numbers for each entity, but possible to interpret with knowledge of the historical context.
The solution in modern English, with the numbers tentatively interpreted, reads:
The soldiers grumble much that the King is of late grown cool towards them and gives them not the encouragement in their addresses as he used to. They complain he is wholly given up to his mistresses, who are no enemies to [the Dutch] nor peace, and consequently dissuade the [King.]
[Colbert,] pursuing the French manufacture with vigour, knows that peace can only advance his designs.
I heard a great man say the [King] said to his brother he wished him not to oppose Madame’s going [to England] because her journey was for the interest of his kingdom, whereupon most do boast of the likelihood of an alliance.
Despite this court gossip relayed by Perwich, Louis XIV and his Comptroller General of Finance, Jean-Baptiste Colbert, did in fact remain belligerent towards the Dutch. The Secret Treaty of Dover was designed to break the Triple Alliance (between England, the Dutch Republic and Sweden) and to get England to attack the Dutch in partnership with France, which happened in 1672.
Louis XIV Crossing into the Netherlands at Lobith, by Adam Frans van der Meulen. Courtesy of the Rijksmuseum, Netherlands.
We also know that Louis managed to prevent his brother, Philippe, from jealously interfering in his wife’s (Madame’s) successful negotiations with her brother, Charles II.
So, the secret message was not about stale croissants, as I feared, but proves to have been worth the bother of being enciphered. Its comments about Louis and his influential mistresses, in particular, could well have offended Perwich’s French hosts. However, it does not suggest Perwich knew anything about the scandalous terms of the forthcoming Secret Treaty with Charles II.