In the 17th century, maintaining secrecy was a monumental challenge. Hiding one's whereabouts was nearly impossible without at least one person being aware of it, and even those who sought refuge—such as Jesuits in priestholes—could not escape the need for food and water. To keep communication under wraps, prisoners resorted to creative methods. Short messages were often smuggled into jail hidden inside eggs, while larger correspondences could be folded into packages small enough to nestle inside a lady's elaborate wig.

In an era where information traveled through more porous channels than today, letters could be delivered by private couriers, sometimes accompanied by spoken messages. Others relied on clandestine networks, including apothecaries and nurses, who moved undetected and could transport sensitive communications. For those using more formal means, letters might be sent to neutral locations under an alias, or sent with a hidden enclosure, where one letter was concealed inside another. For instance, during the English Civil War, Susan Hyde, a royalist spy, used the alias "Mrs. Simburbe" to cloak her identity.

Before the advent of the gummed envelope in the 19th century, letters themselves were folded in complex ways to secure their contents. This intricate method, known as "letterlocking," was so advanced that some designs made it nearly impossible to open a letter without destroying it. Charles I, for instance, could recognize letters from one of his female spies simply by the unique fold. Meanwhile, Oliver Cromwell’s spymaster, John Thurloe, established a "Black Chamber," a covert operation dedicated to intercepting, transcribing, and resealing letters before they were sent on their way.

Even intercepted letters often contained hidden messages, using substitution ciphers or invisible ink to obscure their true meaning. Sometimes, seemingly innocent phrases like "our trade is slow" were codes indicating that the sender was under surveillance. The art of keeping secrets was not only a matter of survival—it could ultimately determine the outcome of wars and the balance of power.