Let’s say you’re an operative with the CIA. Your agents are filing intelligence briefs to you in text documents. But how do you know that the document’s text hasn’t been tampered with somehow by someone malicious who got their hands on it?

You don’t. But researchers have figured out a way to embed encryption into typed blocks of text on paper. Their secret weapon? Helvetica and Times New Roman.

You read that right. Fonts have been transformed into encryption tools. A new paper by a group of researchers at Columbia University details a method for making tiny changes to fonts that the human eye can’t detect but that look entirely different to a computer vision algorithm. A demo of the technology, dubbed “Fontcode,” shows how they were able to embed the secret message “Hello World!” into a paragraph taken from The Lord of the Rings.

How does it work? The researchers use an algorithm from previous research that can slowly shift letterforms from one typeface to another to make tiny changes in the shape of every letter that the human eye can’t detect. That could make an “h” slightly thicker in the stem,” or the curve of a “j” slightly sharper.

Once they had these “perturbed” letters, the researchers could make 52 variations of each letter. Each of the 52 variations corresponds to every other lowercase and capital letter in the alphabet (and theoretically every numeral and punctuation mark as well). These 52 variations for each letter go into what the researchers call a “code book” that helps the computer match the perturbed letter it sees with the secret letter it’s encoding.

To make the system work, the researchers used a deep learning neural network to train a computer vision algorithm to identify the slight variations in each letter correctly.

It’s a clever use of machine learning, one that has immediate applications in security. The researchers’ code book and machine learning algorithm could be used to ensure that documents haven’t been tampered with by embedding an authentication code–or some kind of secret message–into the font itself. Then, when someone receives the document, they can use the decoding algorithm to ensure the message is still there. If it’s not, then someone has somehow altered the document.