How people have sent secret messages throughout history, from invisible ink to coded hairstyles and laundry

James PasleyMar 2, 2023, 04:16 IST

William Abel, a mail censor at Iowa's State Prison, Fort Madison, using a hot iron to bring out a secret message written in lemon juice on a letter.Three Lions/Getty Images

Humans have had a long history with sending secret messages.
The practice dates back to ancient Greeks tattooing a servant's head to a modern-day employee stealing corporate secrets.

Secret messages have been sent for thousands of years.

Folklore tells of enslaved people using methods like hair braiding to secretly communicate without their slave owners knowing, while old women have hung laundry or knitted the routes of trains during wars to help resistance fighters.

More recently, prisoners and employees have tried to break rules and laws — and even put out hits on staff — by sending secret messages.

Here are a number of ways people have sent secret messages over the centuries.

One of the first secret messages in history reportedly traveled on a servant's skull in 513 BC. Greek tyrant Histiaeus shaved his servant's head before tattooing a secret message — that the Persian army led by King Darius was planning an attack — to the man's head.

The war council of King Darius on a Greek vase.Culture Club/Getty Images

One of the first secret messages in history reportedly traveled on a servant's skull in 513 BC. Greek tyrant Histiaeus shaved his servant's head before tattooing a secret message — that the Persian army led by King Darius was planning an attack — to the man's head.

Around the 16th century, folklorists believe enslaved people in Colombia may have used hair braiding to hide secret messages. For instance, if an enslaved person wanted to escape they would tie their hair into thick, tight braids close to the scalp with the ends tied into buns.

Ziomara Asprilla Garcia of Colombia braids brightly colored stands into a woman’s hair as others look on in 2011.Matt McClain/For The Washington Post/Getty Images

Around the 16th century, folklorists believe enslaved people in Colombia may have used hair braiding to hide secret messages. For instance, if an enslaved person wanted to escape they would tie their hair into thick, tight braids close to the scalp with the ends tied into buns.

Rumors have lingered that enslaved people in America used quilts to send messages to help escapees using the Underground Railroad, although it's difficult to verify these claims.

A portrait of abolitionist Harriet Tubman with her family in 1887.MPI/Getty Images

Rumors have lingered that enslaved people in America used quilts to send messages to help escapees using the Underground Railroad, although it's difficult to verify these claims.

During the American Revolution, a woman named Ann Strong who lived in Long Island reportedly used laundry to send coded messages to spies working for George Washington, who was then Commander in Chief of the Continental Army.

A woman hangs laundry in 1900.Hulton-Deutsch Collection/Corbis/Getty Images

During the American Revolution, a woman named Ann Strong who lived in Long Island reportedly used laundry to send coded messages to spies working for George Washington, who was then Commander in Chief of the Continental Army.

At the same time, invisible ink played a role in secret messages being sent to Washington. According to a letter from 1776, one type of invisible ink was invented by an English doctor named Sir James Jay, the brother of Supreme Court Chief Justice John Jay.

An engraved portrait of American politician, governor, and first Chief Justice of the Supreme Court John Jay in his robe in the late 1700s.Kean Collection/Getty Images

At the same time, invisible ink played a role in secret messages being sent to Washington. According to a letter from 1776, one type of invisible ink was invented by an English doctor named Sir James Jay, the brother of Supreme Court Chief Justice John Jay.

During the American Civil War, Elizabeth Van Lew, a well-off woman from Virginia, secretly worked for the Union. She brought prisoners food and medication, as well as sneaking messages into books and in a secret compartment of a custard dish.

Elizabeth Van Lew, an American abolitionist who operated an anti-Confederate spy ring during the Civil War, offers shelter to escaped secret agents of the Federal army in a secret room of her house in Richmond, Virginia.Culture Club/Bridgeman via Getty Images

During the American Civil War, Elizabeth Van Lew, a well-off woman from Virginia, secretly worked for the Union. She brought prisoners food and medication, as well as sneaking messages into books and in a secret compartment of a custard dish.

During World War I, both the CIA and the Germans came up with their own concoctions for invisible ink.

The use of ultraviolet light reveals the use of invisible ink on a letter.Bettmann/Getty Images

During World War II, knitting had its day as a form of covert messaging. In German-occupied Belgium, the resistance had women who lived beside railway lines to knit the order of trains into the clothing to keep track of the enemy's movements.

An 80-year-old woman, who lost five of her six sons during World War II, knits in 1938.Maeers/Fox Photos/Getty Images

During World War II, knitting had its day as a form of covert messaging. In German-occupied Belgium, the resistance had women who lived beside railway lines to knit the order of trains into the clothing to keep track of the enemy's movements.

And at least one Belgian woman used invisible ink on her back to transport information during the war.

A Belgian female spy has invisible ink on her back in 1940.Bettmann/Getty Images

Invisible ink was used again in 1943 to send what appeared to be a love letter from a woman in Kraków to a man in Romania, but it was actually providing information about concentration camps.

A reproduction of a postcard used to send an invisible letter about Auschwitz, the Nazi concentration camp.STR New/Reuters

Invisible ink was used again in 1943 to send what appeared to be a love letter from a woman in Kraków to a man in Romania, but it was actually providing information about concentration camps.

The less-sophisticated method of lemon juice had its day, too. Two spies, a German man named Carl Muller and an English man named John Hahn, reported the movements of British Troops by writing to their government from Britain using invisible ink made with lemon juice.

William Abel, a mail censor at Iowa's State Prison, Fort Madison, — not Carl Muller or John Hahn — using a hot iron to bring out a secret message written in lemon juice on a letter.Three Lions/Getty Images

The less-sophisticated method of lemon juice had its day, too. Two spies, a German man named Carl Muller and an English man named John Hahn, reported the movements of British Troops by writing to their government from Britain using invisible ink made with lemon juice.

There was even one instance where four imprisoned Polish Girl Guides in a concentration camp called Ravensbruck used their urine to send secret letters.

Women working at Ravensbruck concentration camp during World War II.ADN-Bildarchiv/ullstein bild/Getty Image

It was also during World War II that the Germans started using a technique called microdots. J. Edgar Hoover said it was "the enemy's masterpiece of espionage."

These three microdots, magnified several times, featured as exhibits in the spy trial at the Old Bailey in London on March 22, 1961.AP

In 1968, American prisoners of war captured by North Korean forces started sending not-so-subtle messages by raising their middle fingers. It was a way to show their rebellion and to keep their humor.

Men of the captured crew of the USS Pueblo subtly raising their middle fingers in a photograph taken in North Korea.Bettmann/Getty Images

In 1968, American prisoners of war captured by North Korean forces started sending not-so-subtle messages by raising their middle fingers. It was a way to show their rebellion and to keep their humor.

Back in America, the secret messages continued. Prisoners began to use them, though their messages were often more advanced than using invisible ink mae of milk, lemon, or urine.

Col. Mark Adger/Business Insider/Mark Abadi

Back in America, the secret messages continued. Prisoners began to use them, though their messages were often more advanced than using invisible ink mae of milk, lemon, or urine.

In 1999, there were reports that scientists had taken the idea of microdots and used it to hide secrets in DNA. This strategy was more reflective of what scientists were capable of doing rather than signaling the future of hidden messages.

Dr. Maurice Hugh Frederick Wilkins, 46, of Greenwich, England, stands with a model of a DNA molecule during a news conference in the New York office of the Sloan-Kettering Institute for Cancer Research.Anthony Camerano/AP

In 1999, there were reports that scientists had taken the idea of microdots and used it to hide secrets in DNA. This strategy was more reflective of what scientists were capable of doing rather than signaling the future of hidden messages.

As for modern secret messages, most of it takes place on computers. A common way to do it is to hide messages in digital photographs. In 2018, a low-resolution sunset made headlines when a GE employee tried to hide corporate secrets in emails he sent to himself with nothing but the photo attached.

General Electric logo displayed on a phone screen and a binary code displayed on a screen are seen in this illustration photo taken in Krakow, Poland on January 15, 2023.Jakub Porzycki/NurPhoto via Getty Images

As for modern secret messages, most of it takes place on computers. A common way to do it is to hide messages in digital photographs. In 2018, a low-resolution sunset made headlines when a GE employee tried to hide corporate secrets in emails he sent to himself with nothing but the photo attached.

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How people have sent secret messages throughout history, from invisible ink to coded hairstyles and laundry

One of the first secret messages in history reportedly traveled on a servant's skull in 513 BC. Greek tyrant Histiaeus shaved his servant's head before tattooing a secret message — that the Persian army led by King Darius was planning an attack — to the man's head.

Around the 16th century, folklorists believe enslaved people in Colombia may have used hair braiding to hide secret messages. For instance, if an enslaved person wanted to escape they would tie their hair into thick, tight braids close to the scalp with the ends tied into buns.

Rumors have lingered that enslaved people in America used quilts to send messages to help escapees using the Underground Railroad, although it's difficult to verify these claims.

During the American Revolution, a woman named Ann Strong who lived in Long Island reportedly used laundry to send coded messages to spies working for George Washington, who was then Commander in Chief of the Continental Army.

At the same time, invisible ink played a role in secret messages being sent to Washington. According to a letter from 1776, one type of invisible ink was invented by an English doctor named Sir James Jay, the brother of Supreme Court Chief Justice John Jay.

During the American Civil War, Elizabeth Van Lew, a well-off woman from Virginia, secretly worked for the Union. She brought prisoners food and medication, as well as sneaking messages into books and in a secret compartment of a custard dish.

During World War I, both the CIA and the Germans came up with their own concoctions for invisible ink.

During World War II, knitting had its day as a form of covert messaging. In German-occupied Belgium, the resistance had women who lived beside railway lines to knit the order of trains into the clothing to keep track of the enemy's movements.

And at least one Belgian woman used invisible ink on her back to transport information during the war.

Invisible ink was used again in 1943 to send what appeared to be a love letter from a woman in Kraków to a man in Romania, but it was actually providing information about concentration camps.

The less-sophisticated method of lemon juice had its day, too. Two spies, a German man named Carl Muller and an English man named John Hahn, reported the movements of British Troops by writing to their government from Britain using invisible ink made with lemon juice.

There was even one instance where four imprisoned Polish Girl Guides in a concentration camp called Ravensbruck used their urine to send secret letters.

It was also during World War II that the Germans started using a technique called microdots. J. Edgar Hoover said it was "the enemy's masterpiece of espionage."

In 1968, American prisoners of war captured by North Korean forces started sending not-so-subtle messages by raising their middle fingers. It was a way to show their rebellion and to keep their humor.

Back in America, the secret messages continued. Prisoners began to use them, though their messages were often more advanced than using invisible ink mae of milk, lemon, or urine.

In 1999, there were reports that scientists had taken the idea of microdots and used it to hide secrets in DNA. This strategy was more reflective of what scientists were capable of doing rather than signaling the future of hidden messages.

As for modern secret messages, most of it takes place on computers. A common way to do it is to hide messages in digital photographs. In 2018, a low-resolution sunset made headlines when a GE employee tried to hide corporate secrets in emails he sent to himself with nothing but the photo attached.