Hidden in plain sight: an undeciphered letter from Louis XIV’s France

The idea behind a cipher is simple: it's a method to deliberately disguise written information by rearranging and/or replacing its characters with other ones. Text is systematically encrypted so that no-one apart from the intended recipient – who has the 'key' to decipher the message – can read it.

We have thousands of examples of cipher being used in early modern records written by English diplomats or intercepted by English spies (such as the letters which brought about the downfall of Mary Queen of Scots). Many of them were decrypted at the time of receipt or capture, and we can still read the deciphered message where it has been added between the lines of the original letters or attached to the original one.

In some cases, the decryption has not been recorded on the letter itself, but we have the cipher key used by the particular diplomat.

Mysteriously, in the case of the letter below, sent by William Perwich in Paris to Lord Arlington in England on 9 April 1670, there is no decryption. Although we hold keys to Perwich’s later numerical ciphers, we do not hold one for the unusual alphabetical code he employed in this and his next letter.

Paris Apr: 9th. 1670 S[tyle] N[ouveau]

May it please your Lo[rdshi]pp.

Wee are in hourly expectac[i]on of Mr Godolphin’s arrival here. Yesterday the King came to Paris much to the Burgers satisfaction, visited several Churches & was present in ye afternoon at ye Ceremonious babtisme of Madams young daughter the duchesse de Valois, who had ye Dolphin to Godfather, & ye Queen with Mad[amoise]lle to GodMothers.

a Guernsey Merchant dying Lately in this place his goods were seized on by droit d’aubeine but upon my L[or]d Ambassadeur complaint we are in hopes ye King will dispence with what our forme[r] Treatys have granted to the subjects of England.

Perwich was a minor diplomat working for the English government at the court of the King of France, Louis XIV. Henry Bennet, Lord Arlington, was a close advisor of Charles II and one of his Secretaries of State.

Perwich’s correspondence was considered significant enough to be transcribed and published in 1903. Ominously for future historians, however, the editor of his letters didn't even mention that some sections were in cipher, excusing herself by saying: ‘Where the original is in cipher, only the deciphered portions have been printed, in accordance with long-established usage’.

The year 1670 was a busy one for Louis XIV, with conflicting negotiations taking place through the courts of Europe as the different powers jockeyed for position. In May, the month after this letter was despatched, Louis sent his sister-in-law (Charles II’s sister Henriette) over to Dover to negotiate between France and England. What very few people knew was that she was also authorised to sign what is now known as the Secret Treaty of Dover between the two monarchs, whereby Charles agreed to convert to Catholicism in return for a large payment from the French king. Such an agreement, if known, would probably have led to Charles losing his throne as his father (Charles I) and, subsequently, his brother (James II) did.

Was Perwich commenting on this Treaty in his ciphered letter, even though he was not supposed to know anything about it?

Another thing I must not omit to impart to your Lo[rdshi]pp I hear from good hands That among other restorac[i]ons made de part et d’autre between the French & Algerins upon their late Treaty of Peace, ye former putt again into the Pirats hands an English Vessell they had retaken certainly such an inhumane proceeding cannot bee owned by Civil or Christian Laws.

Now ye Kings voyage draws nigh & ye Courtiers prepare in earnest. his Troops are upon the place by this time; but (his inclinations whatever they bee) its generally believed his intentions are not to doe any thing but view his new Fortifications w[hi]ch I am told by many of judgement are the best in Europe & am promised the models but not without expence.

I am reasonably certain that the type of cipher used by Perwich in this letter is a transposition cipher, which (to misquote the late, great Eric Morecambe) displays all the correct letters but not necessarily in the correct order. The alternative is a substitution cipher where each letter, pair of letters, place or person is replaced by a number or another symbol.

In Perwich’s letter, the letter ‘e’ is the most frequent letter - as it would be in English - and the writer has used typical abbreviations instead of letters in some instances, e.g. ‘ye’ for ‘the’ and ‘yt’ for ‘that’. The layout and the the date of the letter suggest it is based upon a method published by one of Arlington’s acquaintances, Sir Samuel Morland, just four years earlier.

Morland was a double agent and a diplomat but, above all, a scientist and mathematician. Along with many other mechanical inventions, he developed machines for the Post Office to open, copy and reseal letters (mostly destroyed in the Great Fire of London). In 1666, he published ‘A New Method of Cryptography’, which described a cipher which he believed to be so tough to crack that sharing its general principles would not make it vulnerable without access to the sender’s personal key. He claimed that the approach could result in over 10,000 quadrillion (UK scale) different transpositions for a grid of just 81 letters. To find out how his cipher operated, see Katherine Ellison’s video.

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There are a few anecdotes of Morland’s cipher being used by diplomats corresponding with Arlington in the late 1660s and we have three of their personal cipher keys in SP 106/6, but none for Perwich. The experience does not appear to have gone down well with them. In particular, Sir William Temple, who was representing the Crown in the Netherlands, became exasperated at his inability to read the ciphered letter he had received from Arlington and ended up working with his Dutch counterpart to attempt to decrypt the very text which was being kept secret from the latter.

We know Perwich was interested in trying out this cipher because in 1669, Temple wrote that ‘For Sir Samuel Morland's Cypher… Mr. Perwick wrote from France for a Tryal between us; therefore I should be glad you would please to send your exact Rule, as you now use it, with a good quantity of the ruled papers, by some safe hand.’

There are a number of cipher-cracking apps available online and I attempted to put my transcription of Perwich’s ciphered text through one but was only able to generate further gobbledygook. Perhaps it really is indecipherable without his personal key. I do hope someone does manage to crack his message eventually though, even if it turns out only to be a complaint about the food at Versailles rather than background to the Secret Treaty of Dover.

The ciphered text

If you'd like to try and decipher it yourself, here is my attempt to transcribe Perwich's ciphered text in full.

I've chosen to present it in a table so that the positioning of the 2- and 3-character abbreviations and numbers is clear – since this could be crucial to understanding it:

Further reading

M. Beryl Curran (ed.), The Despatches of William Perwich, English Agent in Paris, 1669-1677 (Camden Record Society, 3rd series, vol.V, 1903)

Katherine Ellison, Secret Writing in the Long Eighteenth Century (Cambridge University Press, 2022), plus video on Morland’s method

R. Hutton, ‘The Making of the Secret Treaty of Dover, 1668-1670’, The Historical Journal, Vol.29 no.2 (Jun. 1986), pp.297-318

Samuel Morland, ‘A New Method of Cryptography’, (London, 1666) - full copy on the Internet Archive, more readable selected pages available from Folger Shakespeare Library Digital Collections

Ruth Selman, ‘A Most Lamented Princesse’: an English princess at Versailles‘ (The National Archives Blog, 11 Aug 2016)

S. Tomokiyo, ‘Samuel Morland's "New Method" Used for Charles II's Ambassadors’, (Cryptiana website, 2023)