Secret diplomatic message deciphered after 350 years

Summary

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 decryp...