Oliver Cromwell’s postal spies: intercepting letters and codebreaking in the General Post Office

Early intelligence gathering

Until the 1640s, intelligence gathering in the British Isles was a matter of individual spies reporting to their own spy chiefs – men such as Sir Francis Walsingham, the Earl of Essex, and William Cecil and his son Robert. As rivals competing for the favour of their monarch, cooperation was not their first instinct.

Operations tended to be limited to the infiltration of groups that might harbour potential enemies, but also the occasional interception of letters exchanged between known or suspected conspirators. And that was often a tricky task.

Before any agent could attempt to read a message contained within a letter – which might be protected through ciphers, codes or invisible inks – they had to open it, preferably without damaging it.

Long before the invention of gummed envelopes, letters were as intricate as origami birds. The paper on which the message was written was folded into its own letter-packet. Security devices included wax seals and ‘locks’ – pieces of paper threaded through the letter-packet that would rip upon opening, betraying the fact that the letter had been compromised. Great skill was required to deal with these anti-tamper devices; that might mean mending the damage caused by opening a letter or, when that was impossible, counterfeiting all or part of it.

Breaking through the wax seal

The first security measure encountered was typically the seal – a blob of molten wax into which a design was pressed, simultaneously identifying the sender and preventing the letter from being easily opened. Such seals could be removed invisibly if treated with care. In the second century AD, the writer Lucian recommended using a hot needle to prise off a wax seal.

By the 16th century, though, the pliable beeswax-and-turpentine mixture used for seals in Lucian’s time had been replaced by a more secure wax. This brittle substance, prone to shattering, contained hardening substances such as gum arabic – obtained from the sap of acacia trees – or shellac, a resinous substance secreted by a species of Asian bug.

The 16th-century Italian polymath Girolamo Cardano suggested warming the wax before using a horse-tail hair to slice under the seal and detach it in one piece. This was, of course, a very delicate operation, and Cardano recommended that less dexterous operators counterfeit the seal instead.

The pliable beeswax-and-turpentine mixture used for seals in Lucian’s time had been replaced by a more secure wax. This brittle substance, prone to shattering, contained hardening substances such as gum arabic or shellac

To do so, a new seal might be created from scratch, copying the original design and casting a facsimile in brass. This took time and no little skill, though, and would be done only for suspects responsible for high levels of correspondence. For seals newly or only occasionally encountered, a fragile plaster-like cast could be used to directly mould a new wax seal. This process required a sample of the seal that was clear enough to be duplicated.

In 1596, counter-espionage agent Arthur Gregory complained to Robert Cecil that “among so many seals… there is no choice of one perfect print”. Gregory would be best able to copy a ‘bare-seal’, formed by pressing a seal-stamp into molten wax on the outside of the letter-packet. The other type, a ‘covered’ or ‘papered’ seal – created by dripping molten wax between the two pieces of paper to be joined – was next to impossible to counterfeit.

A counterfeiter’s craft – and curse

Once a suitable seal had been identified, a cast could be made – which also required quite some preparation and skill. In his 1558 book of secrets, Magiae Naturalis (Natural Magick), polymath Giambattista della Porta suggested that, before attempting to make a cast, the counterfeiter should first “compass the seal about with wax, that the matter [the casting material] run not about”.

This would prevent it from staining the surrounding paper, revealing the counterfeiter’s work. He also advised that the original seal be “anointed with oil” to prevent the casting material from sticking to it. The cast, once made, carried a negative image of the seal, which could then be used to cast a new seal from wax.

When time was of the essence, a counterfeiter’s choice of casting material was all-important. Nothing was so likely to put the wind up conspirators as a letter taking 12 days to reach its recipients when it ought to have been there in four. Excessive delay in delivery of a letter was a tell-tale indicator of possible interception.

To avoid such revealing delays, Arthur Gregory aimed to formulate a casting compound that dried at speed and to a metallic hardness – with unfortunate consequences. In February 1586, while working to perfect his ‘sealing metal’, he wrote to Walsingham: “I prepared a singular piece of the usual metal to show the same to your Lordship, but a sudden swelling in one of my eyes did prevent me.” A decade later, he connected his activities with recent afflictions: “With the making of metal this last night I have gotten an unsteadiness in my hand which will not leave me for a few days.”

Though it’s impossible to be sure – Gregory failed to leave behind a recipe book for posterity – it seems likely that the formula he worked to perfect was inspired by della Porta, who used toxic ingredients such as ceruse (white lead), verdigris (copper carbonate) and quicksilver (mercury), any one of which might have caused Gregory’s symptoms.

The General Post Office opens for business

As well as counterfeiting seals, intelligence agents like Gregory also sought to master the imitation of handwriting and particular letterlocking styles, translation, cryptanalysis and myriad other tasks.

During the time of Walsingham and the Cecils, intelligence operatives worked on an ad-hoc basis – and often alone. Their efforts sometimes delivered tangible results – for example, foiling the Babington Plot of 1586, a scheme to assassinate Elizabeth I and put Mary, Queen of Scots on the throne.

• Read more | Alternate history: what if the Babington Plot to assassinate Elizabeth I had succeeded?

But the lack of a centralised and resourced surveillance system could allow conspiracies to blossom unchecked. After the execution of Charles I in 1649, the nature of intelligence gathering changed. Oliver Cromwell, ruling as Lord Protector from 1653, was aware that the British Isles was shot through with supporters of Charles Stuart, son of the executed king.

So he created a system of surveillance with the aim of controlling the primary method of communicating information over distance: the letter. In 1655, Cromwell installed his secretary of state and head of intelligence, John Thurloe, as postmaster general of a new institution, the General Post Office.

His reasoning became clear in 1657, when An Act for Setling the Postage of England, Scotland and Ireland stated that the founding of “one general Post Office” would allow the Commonwealth to “discover and prevent many dangerous, and wicked designs… the intelligence whereof cannot well be communicated, but by letter”. Cromwell’s Post Office, then, was created not only to deliver letters but to intercept them.

Precursor to the Post Office

The idea of using a postal monopoly for the comprehensive surveillance of communication was nothing new. In the 1630s, Alexandrine, Countess of Taxis, was described as “a troublesome busy Woeman, [who] will open letters, [and] hath counterfait seales of publicque Ministers”. Alexandrine’s family held the monopoly on postal services throughout the Holy Roman Empire – in other words, most of western Europe.

Between 1628 and 1646, while she minded the family business until her young son came of age, she developed a system by which letters were methodically opened and inspected for valuable information before being resealed and sent on their way, their recipients (generally) none the wiser. The information was then sold to the highest bidder. Alexandrine’s operatives worked in a back room of what we would now call a post office.

Similarly secreted offices known as cabinets noir (‘black chambers’) later popped up in Paris and Vienna. The interception of letters was not uncontroversial. When Stuart diplomat and art broker Balthazar Gerbier realised that someone in Alexandrine’s office was opening and reading his letters, he complained of the “violation of his sacred letter-packets”.

And in 1642, Lucius Cary, 2nd Viscount Falkland and Charles I’s secretary of state, argued that the opening of private mail was “a violation of the law of nature that no qualification by office could justify a single person in the trespass”.

Inside the Black Chamber

For Cromwell, however, the needs of the state justified riding roughshod over private delicacy. His postmaster general, John Thurloe, set up his Black Chamber in the ‘Queen’s Closet’ of the palace of Whitehall, London. In this room, decorated with gold leaf, his men processed all of the official mail sent into and out of the capital, as well as that between England and the continent.

The Black Chamber aimed to provide continuous surveillance, and employed several operatives on what was effectively a production line that sought to systematically interrogate suspect letters. The work was conceptually simple but in practice complex, demanding and time-consuming.

• Read more | The dark truth about Oliver Cromwell

The volume of post was too large for everything to be opened, so operatives concentrated efforts on letters for which addressees, recipients, handwriting or seals were already on their watch list. Once identified, these were opened and read, and any suspect contents copied, translated, deciphered or decoded. They were then resealed and sent on to their recipients to avoid spooking any conspirators before enough evidence had been gathered to catch them all.

By the time of the Restoration, Thurloe had, understandably, made his exit, but the Black Chamber itself remained as part of the fabric of government. By then, its operatives were virtually civil servants.

The regime may have changed, but two of the more skilled employees, Isaac Dorislaus and Samuel Morland, remained in office to serve the new monarch, Charles II. (This is perhaps all the more surprising when you consider that Dorislaus’s father had been listed as one of the regicides, accused of involvement in the sentencing and execution of Charles I, and had been murdered by royalists in May 1649.)

Lost secrets of Charles II's counter-espionage

Morland, a natural entrepreneur in the ‘dangerous trade’, was more than happy to boast of his own victories – presumably safe in the knowledge that they were unlikely to be contradicted – especially when lobbying for cash from the restored king. He claimed that he had saved Charles’s life through the “dexterous opening and Sealing up of one letter Sealed carefully with a Wafer”, and that “by this Art of opening letters & Seals, and copying out dispatches in a moments time &c. were very great services done to King Charles 2”.

Morland was something of a polymath. A keen inventor, he not-so-quietly took over the role of early modern Q earlier filled by Arthur Gregory. In 1664, he was granted an audience with Charles II, during which he demonstrated a series of “models in little of several engines and utensils” he had designed to help automate the counterfeiting skills on which the Black Chamber relied. Finding it impossible to distinguish counterfeits from the original letters, the king ordered that Morland’s methods be put into practice.

Like Bletchley Park, the 17th-century Black Chamber drew together an array of talented counter-espionage experts and supported them with cutting-edge technologies. Unlike Bletchley, however, neither the Black Chamber nor any of its devices survived to be investigated further.

When writing his memoirs in 1689, Morland recounted how another of his innovations, a machine that could copy entire documents in mere minutes, was used “at the General Post Office until at the Fire of London the apparatus was destroyed and was not set up again”.

Perhaps it’s apt that this covert unit, and the tools it used to such effect in counterespionage, effectively vanished – virtually without trace.

Nadine Akkerman is professor of early modern literature and culture at Leiden University. Pete Langman is a writer and academic. They are the authors of Spycraft: Tricks and Tools of the Dangerous Trade from Elizabeth I to the Restoration (Yale University Press, 2024)