Collection blog

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

The National Archives holds many documents that can be hard to read – from 20th
century letters in messy handwriting, to poorly-spelt 19th-century copperplate,
to the somewhat bizarre letter forms of medieval court hands. Letters written in
cipher, however, are the trickiest of all.

Published 4 August 2025 by [Ruth
Selman](https://www.nationalarchives.gov.uk/people/ruth-selman/)

![A page from a volume showing an equally-spaced sequence of jumbled letters above normal correspondence.](https://www.nationalarchives.gov.uk/media/images/ciphered-letter-blo.e82430ef.fill-1800x720.format-jpeg.jpegquality-60.bgcolor-fff.jpg) 

About this image 

Letter from William Perwich to Lord Arlington, 9 April 1670. Catalogue
reference: [SP 78/129,
f.180r](https://discovery.nationalarchives.gov.uk/details/r/C4539746)

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.

      

![List of the letters of the alphabet with multiple unusual symbols below each one, and months of the year listed.](https://www.nationalarchives.gov.uk/media/images/mar_cohLxye.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg)

Extract from a cipher key used by Mary Queen of Scots around the year 1586.
Catalogue reference: SP 53/22 f.1

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.

      

![Two pages of handwritten text stuck into a volume, beginning with seven lines of equally spaced, seemingly random letters.](https://www.nationalarchives.gov.uk/media/images/per.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg)  Toggle partial transcript

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.

Letter from William Perwich to Lord Arlington, 9 April 1670 ([New
Style](https://simple.wikipedia.org/wiki/Old_Style_and_New_Style_dates)).
Catalogue reference: [SP 78/129,
f.180r](https://discovery.nationalarchives.gov.uk/details/r/C4539746)

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?

      

![Two pages of handwritten text stuck into a volume, ending with 15 lines of equally spaced, seemingly random letters.](https://www.nationalarchives.gov.uk/media/images/per_B82wYaC.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg)  Toggle partial transcript

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.

Perwich's letter continues.

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](https://archive.org/details/bim_early-english-books-1641-1700_a-new-method-of-cryptogr_morland-sir-samuel_1666/mode/2up)’,
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](https://drupal-s3fs-prod.s3.eu-west-1.amazonaws.com/resources/academic/7416/6669/1116/Video1Morland.mp4).

Image 1 of 2

![](https://www.nationalarchives.gov.uk/media/images/mor.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg)

Samuel Morland’s cipher key adapted for Sir Bernard Gascoigne, 1668. Catalogue
reference: [SP 106/6, item
15](https://discovery.nationalarchives.gov.uk/details/r/C3677736)

Image 2 of 2

![](https://www.nationalarchives.gov.uk/media/images/mor_iGzEdVl.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg)

Close-up of Samuel Morland’s cipher key.

Previous      Next

![](https://www.nationalarchives.gov.uk/media/images/mor.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg) Image 1  of 2  ![](https://www.nationalarchives.gov.uk/media/images/mor_iGzEdVl.max-1200x1200.format-jpeg.jpegquality-60.bgcolor-fff.jpg) Image 2  of 2

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](https://discovery.nationalarchives.gov.uk/details/r/C3677736), 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:

Transcript of the ciphered section of Perwich's letter

|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| R | m | d | g | o | r | p | w | a | r | q | d | s | m | r | o | n | d | s | u | r | c | Q | R. |  |  |  |
| i | g | e | e | d | e | w | w | r | i | r | o | C | h | s | s | a | r | e, | u, | U, | n | d | p | a | r | q |
| r | s | a | u | e | m | n | o | e | s | n | r | a | a | y | t | s | o | t | o | a | n | p |  |  |  |  |
| e | i | w | o | r | o | e | h | p | s | e | u | y | e | a | h | m | I | s | p | i | n | G |  |  |  |  |
| s | o | r | r | s | p | u | a | a | u | c | k | d | r | d | h | g | u | o | n | p | q | n |  |  |  |  |
| g | f | d | a | s | l | p | r | e | a | h | n | u | d | t | i | o | r | f | m | c | d | p |  |  |  |  |
| r | l | s | g | e | a | t | e | e | d | m | o | a | a | o | m | i | n | h | o | e | Q |  |  |  |  |  |
| m | o | e | i | e | i | m | e | u | c, | a | s | m | o | o, | 40, | f | g | o | 6 | R | o | m |  |  |  |  |
| e | r | u | t | e | e | m | e | s | p | a | e | i | t | r | o | r | s | n | b | d | m | p | o |  |  |  |
| L | g | i | n | h | e | s | n | n | 96 | f | p | h | a | b | t | o | a | i | o | n | m | o |  |  |  |  |
| b | e | g | e | s | h | i | e | o | 60 | u | yt | e | e | s | t | f | w | k | d | o |  |  |  |  |  |  |
| m | t | & | m | a | n | h | o | e | ye | n | s | e | r | i | o | g | y | s | t | 40 | o | 910 |  |  |  |  |
| u | a | ym | e | s | i | o | n | & | e | a | w | n | g | h | n | n | e | i | s | 40 | d | o | r | m | q |  |
| d | n | t | n | d | y | i | s | o | d | f | g | e | I | 61 | i | d | e | r | r | a | m | o | g | b | t | 192 |
| s | u | w | s | n | u | s | s | i | q | r | t | c | d | a | t | p | b | o | d | a | n | d | 96 |  |  |  |
| L | i | l | e | a | e | g | e | n | y | ye | i | n | s | ye | a | a | h | e | e | h | w | a |  |  |  |  |
| s | yt | c | o | c | d | o | e | t | n | i | e | a | i | a | r | s | a | i | e | o | m | d |  |  |  |  |
| o | ye | o | t | i | t | U | s | o | t | n | wt | n | g | y | h | e | u | n | w | f | a |  |  |  |  |  |
| c | h | e | n | h | e | h | r | s | e | u | r | e | s | s | e | o | c | ye | m | t |  |  |  |  |  |  |
| h | c | n | ym | t | s | w | t | e | n | s | e | e | n | h | p | e | r | o | s | m | d |  |  |  |  |  |
| u | k | o | ye | r | h | y | s | 97 | k | g | u | o | n | yt | e | m | s | t | h | a | m | Q | R |  |  |  |
| m | d | p | o | r | m | h | m | a |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |

## 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](https://www.cambridge.org/universitypress/subjects/literature/english-literature-1700-1830/secret-writing-long-eighteenth-century-theories-and-practices-cryptology#resources)

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](https://archive.org/details/bim_early-english-books-1641-1700_a-new-method-of-cryptogr_morland-sir-samuel_1666/mode/2up),
more readable selected pages available from [Folger Shakespeare Library Digital
Collections](https://digitalcollections.folger.edu/bib147537-181583)

Ruth Selman, ‘[A Most Lamented Princesse’: an English princess at
Versailles‘](https://webarchive.nationalarchives.gov.uk/ukgwa/20250613153638/https:/blog.nationalarchives.gov.uk/lamented-princesse-sudden-death-princess-henriette-anne-england/)
(The National Archives Blog, 11 Aug 2016)

S. Tomokiyo, ‘Samuel Morland's "New Method" Used for Charles II's Ambassadors’,
([Cryptiana website](https://cryptiana.web.fc2.com/code/morland.htm), 2023)

## Author

* ![Profile photo for Ruth Selman.](https://www.nationalarchives.gov.uk/media/images/Ruth_S.2e16d0ba.fill-128x128.format-jpeg.jpegquality-60.bgcolor-fff.jpg)

  **[Ruth Selman](https://www.nationalarchives.gov.uk/people/ruth-selman/)**

  Roles
  :   Author

  Early Modern Records Specialist

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