Secret Numbers and Royal Betrayal: Queen Henrietta Maria’s Coded Letters to King Charles I in 1642

How a queen’s secret correspondence helped sustain the Royalist cause as England descended into civil war.

One of the challenges in writing about the English Civil War is the number of ‘rabbit holes’ to distract me. One example is exploring the wealth of secret correspondence between the king and queen. 

In the summer of 1642, England faced catastrophe. King Charles I and Parliament reached an impasse, armies gathered, and Queen Henrietta Maria was in The Hague, working to raise money and secure the weapons her husband desperately needed. The problem was a letter could be intercepted, its contents exposed, and the information used against the Crown.

Henrietta Maria and Charles relied on secret codes or ‘ciphers’. Their letters were more than messages between husband and wife. They were full of information about money, military supplies, diplomatic negotiations and people whose loyalties might determine the outcome of the coming war.

As part of my research into this I bought a copy of the original letters from Queen Henrietta Maria to Charles I, including correspondence written in ciphers. These letters provide a fascinating glimpse into a secret world, hidden behind an apparently meaningless string of numbers.

A secret language of numbers

One historical account describes the introduction of a new cipher in a letter written shortly before King Charles I raised his royal standard at Nottingham on 22 August 1642. The explanation is remarkably precise:

It consists of figures only. The system was more sophisticated than a simple substitution cipher, in which each letter is replaced by a number. According to the description, numbers up to 77 represented the letters of the alphabet, while higher numbers, extending to 379, stood for the names of people and a selection of frequently used words.

These included terms such as money, peace, treaty and rebels. This was a practical solution to a serious problem. A letter discussing a loan, a proposed treaty or a named political ally could be dangerous even if its individual sentences appeared harmless. 

Queen Henrietta Maria's cipher example 

By assigning frequently used names and politically sensitive words their own numerical codes, the correspondents could make their messages harder to interpret. The cipher also contained a secondary system in which a letter combined with a number represented short, common words such as will, be, of and from. Such devices reduced the need to spell out every word individually and made the code more efficient to use.

To an enemy who intercepted the letter without the key, the page looked like little a jumble of figures. To its intended recipient, it could convey a message about the survival of the monarchy.

Why Henrietta Maria needed secrecy

Henrietta Maria was not a queen who could safely leave political affairs to her husband. In 1642, she was in the Dutch Republic, ostensibly to hand over her ten-year-old daughter Mary to her new ‘husband’, William of Orange. 

Her real plan was buy arms and ammunition for the Royalist cause, using - by trying to sell and obtain loans against several of the Crown jewels, as well as the best of her own jewels, smuggled out of England.

She needed to keep Charles informed of her progress, while receiving news from England and coordinating decisions with him. A letter had to travel through several hands before reaching its destination. Couriers could be questioned, ships delayed or searched, and correspondence seized by political opponents.

The secret cipher offered protection, although no system was entirely secure. The more important the information, the greater the incentive for an adversary to break the code.

Cracking the royal cipher

The remarkable feature of the numerical system is that the key used to decipher the queen's letters helped unlock passages in Charles I's correspondence. The historical account explains that some of the king's letters, previously printed in contemporary pamphlets without their coded passages being understood, could be deciphered using the queen's key.

This is an important clue to how the system worked. The two correspondents needed compatible methods of encoding and decoding their messages. Once a complete key had been recovered, it could reveal more than one letter and sometimes more than one correspondent's contribution to the exchange.

A cipher key was a valuable political document, and King Charles carried his with him in his pocket. In the wrong hands, it could expose the meaning of letters that had been considered secure. 

On 22 August 1642, Charles raised his royal standard at Nottingham, formally committing himself to military resistance against Parliament. The king's decision followed months of escalating confrontation. 

King Charles I Raising His Standard at Nottingham Castle, 1642

(Wikimedia Commons)

The raising of the standard was a public declaration of royal authority. Henrietta Maria's coded letters belonged to the secret struggle behind it: the effort to obtain the money, arms and foreign support needed to sustain the king's cause.

The contrast is striking. While Charles made his intentions visible before the people gathered at Nottingham, his queen was engaged in a clandestine campaign of negotiation and procurement, communicating through letters whose most important passages were hidden in numbers.

The human cost behind the numbers

There is something poignant about a letter reduced to figures. Behind every encoded name or reference to money lay a human relationship and a political calculation. A trusted messenger had to carry the paper. Someone had to copy the numbers accurately. The recipient needed the right key, and both correspondents had to hope that no one else had acquired it.

Henrietta Maria's numerical cipher shows a queen working actively to defend her husband's cause, and a royal couple attempting to maintain their partnership when ordinary communication had become perilous. The figures on the page may look cold and impersonal. Their purpose was anything but. They carried the hopes of a monarchy under siege, at the moment when England's political crisis was becoming a civil war.

Tony Riches

Further reading

The British Library, Egerton MS 2619: original letters from Queen Henrietta Maria to Charles I, 1642–1645, including correspondence partly written in cipher.

Mary Anne Everett Green (ed.), Letters of Queen Henrietta Maria, including her Private Correspondence with Charles the First (1857).

The Folger Shakespeare Library catalogue, The Queens Majesties Message and Letter from the Hague in Holland, a contemporary pamphlet published in October 1642.

My new book, Queen Henriette, the second in my Stuart Queens trilogy, wiill be released in spring next year, subject to not falling down more 'rabbit holes' In the meantime I'm please to see the first book, Queen Anna, about Anna of Denmark, doing well, see: https://mybook.to/QueenAnna