Our personal data is among today’s most valuable information currency. It’s often hard to determine what part companies own, what part the government owns, and what part, if any, is entirely our own — provided a third party hasn’t already sold it to someone else.

These intrusions of privacy aren’t new. For centuries, the post office had every intention of reading your mail. Beginning in 16th-century France, the “black chamber” or cabinet noir was set up in governments throughout Europe to scan incoming mail for traitorous or politically useful correspondence. The “black chambers” were run by the government and hidden from the public, even though their presence was obvious to all: letters were opened, copied, their seals re-waxed by specialists.

“In the eighteenth century, there was no expectation of privacy when the postal system was used,” writes John Nagy in Invisible Ink: Spycraft of the American Revolution — a fascinating lens on the Revolutionary War as a war of information, where no correspondence could be sent that wasn’t first encrypted and whichever side hid theirs the best would come out on top. Here are five key ways the British and the Americans duped each other:

INVISIBLE INK

Invisible ink, far from the magical compound pop culture has made it out to be, has been around for ages — a book on invisible writing was published in 1653. Invisible ink involves any acid which will weaken the paper, causing it to darken and burn if heated. Benedict Arnold used it in his traitorous correspondence with the British, and the Americans used it just as often.

Instead of heat, George Washington used a chemical form of agent and reagent, and he was quite often on the verge of running out of one or the other as he moved from camp to camp. When he would write to his supplier requesting more, he would always refer to the solution as “medicine.” Eventually, a lab was set up for the express purpose of making invisible ink for the general.

ENCRYPTION

Codes were perhaps the most common form of encryption, and they ranged from the simple substitution of one letter for one number, or a shifted alphabet, to the diplomatic ciphers practiced by Benjamin Franklin, who chose for his cipher text an appropriately obscure snippet of French prose. Washington used at least four cipher alphabets: one transposition, one substitution, and two varieties of the pigpen cipher, which was derived from Freemasonry. (At the beginning of the nineteenth century, Thomas Jefferson even used a special cipher just for the Lewis and Clark expedition.)

MASKING

For his personal correspondence, British General Henry Clinton would handwrite a regular letter and then create a mask for it, a cutout either in the shape of an hourglass or a paper with little windows in it, which would fit over the original letter to reveal the secret message. Presumably sent separately, the mask, or “cypher paper,” would sometimes not make it to the recipient, causing one friend in England to muddle through a letter from Clinton for “some time, and not until several readings, ere I found there was a secret centre.”

THE DICTIONARY

The most popular mode of encipherment was something both sides had access to: Entick’s Spelling Dictionary. Diplomat John Jay used it heavily to creative elaborate codes with page numbers, dots, and substitutions. With thirteen editions in existence at the time of the war, it was essential that both parties have the same version. Sometimes, the key reference book was less well known: Benedict Arnold used the first volume of the fifth Oxford edition of the Commentaries on the Laws of England for his coded correspondence.

DECEPTION

Washington couldn’t spend as much money on spies as the British, but he could at least trick any spy that might report on the American army: he would ship sand and pretend it was gunpowder, he prepared inflated troop returns to hand off to double agents, he famously had his troops dress as Indians.

In addition to offering a fascinating slice of history, Invisible Ink is also a reminder that access to information was, and remains, a game-changing bargaining chip of power.

Michelle Legro is an associate editor at Lapham’s Quarterly. You can find her on Twitter.