The history of invisible ink is finally out in the open

The World

I have been finding little scrappy bits of seemingly blank paper all over the house lately.

I say "seemingly" because my daughter recently got a "top secret sleuthing kit" for her 8th birthday.

The kit contains two pens. One pen has "invisible ink," which you use to craft your super-secret messages.

You use the other pen to rub over the note, revealing those messages.

My daughter's notes usually say something like: "Can I have a cookie?"

I tend to write back: "Eat your vegetables."

Okay, not exactly Spy vs. Spy, but it did pique my interest.

And then, I received a review copy of a new book called "Prisoners, Lovers and Spies: The Story of Invisible Ink from Herodotus to Al-Qaeda."

“People are always surprised, because they think this is kid's stuff. It is serious business,” says the book’s author Kristie Macrakis.

Her research focuses on the historical intersections between science and espionage.

On a trip to Berlin a few years ago, she was working on a book about the East German secret police, the Stasi.

She'd gotten hints that the Stasi liked to use invisible ink, but she'd never seen an actual method or formula detailed.

Then the archivist handed her a very thin file, tucked in among a bunch of others.

“I opened it up, and my mouth fell open, and I thought, 'Wow, finally,” Macrakis says. It contained invisible ink formulas from the Cold War in the 1970s.

Macrakis hand-copied the whole file, convinced there was no way the archivists would make a photocopy for her.

But she asked anyway, and to her surprise, they did.

A few minutes later, she was making a quick getaway.

“I stuffed the copy of the file in my knapsack and started waltzing down the steps in my sandals. I felt like I was in a Hitchcock movie, you know. I fled down the steps and looked behind me, and they weren't asking for my files back, and I walked outside, thinking I was home free.”

Macrakis brought the Stasi secret ink formula back to Georgia Tech in Atlanta, where she teaches. She found a chemist friend, and together with some students, they recreated the formula through trial and error.

Macrakis was having so much fun with the topic that she decided to write a book about the history of invisible ink and "secret writing:" steganography, to use the proper name.

To be clear, this isn't cryptography. We're not talking codes and ciphers.

We're talking about the Roman poet Ovid giving Dear Abby-style advice about the kind of plant juice that lovers could use to write secret messages.

We're talking about prisoners who have used oatmeal and milk, urine and even semen to write notes.

We're talking about al-Qaeda hiding correspondence in the pixels of a porn film.

You name it, says Macrakis, and it's been tried.

“Just use your imagination. Throughout history, you find the most fascinating ways of concealing secret messages, whether it's in jewelry, or even writing on bodies, or even in a tooth. There are lots of ways of concealing and they're all just fascinating.”

By now, you’re probably thinking back to those days as a kid when you tried this with lemon juice. You take the juice, dip a toothpick or other sharp object in it, and then write on a piece of paper.

Then you hold the paper over a flame and the message appears. Primitive, but it works.

In fact, in writing the book, Macrakis uncovered the story of the "Lemon Juice Spies" — a group of Germans in Britain who were caught using lemon juice to try to send secret messages during World War I. She even found one of the actual lemons used as evidence against them in a box.

“I mean, I looked at it, and I thought, ‘Wow, a 100-year-old lemon.’ And this innocuous lemon meant that about a dozen German spies were shot one by one in the Tower of London.”

Of course, the invisible ink cat-and-mouse game between the Germans and the Brits continued into WWII as well.

During that war, the Brits caught a Norwegian named Nickolay Hansen.

“At first, he told a bunch of lies, and they asked him about secret writing, and he said he didn't use any,” Macrakis relates. “And finally, it came out that he hid a little baggie with invisible ink in his molar. Hopefully, he already had a hole in his tooth. Hopefully, they didn't drill a new hole in his tooth to hide the secret ink in it.”

Macrakis also writes about Britain's mass intercept of war-time mail.

Teams of women in Bermuda would sift through letters heading from the US to Britain, searching for secret messages.

Draw a pretty straight line, she says, to Edward Snowden's revelations about the NSA.

“After WWI, for example, the British claimed they had a grip on the world's correspondence,” Macrakis says. “Similarly, that's what the NSA wants to do with blanket electronic surveillance. It's really the same story of secret writing 100 years before with mail interception, just with digital methods.”

Everything old is new again, as the saying goes.

There's one mystery, though, that Macrakis hasn't been able to figure out.

Sixteenth-century Italian scholar and polymath Giambattista della Porta suggested using a mix of alum and vinegar to write secret messages on an egg.

Macrakis says she's tried and tried, but can't get it to work.

She's says she'll soon launch a contest offering 200 bucks to anyone who can do it.

Get cracking

Will you help our nonprofit newsroom today?

Every week, more than 2 million listeners tune into our broadcast and follow our digital coverage like this story, which is available to read for free thanks to charitable contributions from listeners like you. But less than 1% of our audience supports our program directly. From now through the end of the year, every gift will be matched dollar for dollar by a generous donor, which means your gift will help us unlock a $67,000 challenge match.

Will you join our growing list of loyal supporters and double your impact today?