Steganography

Steganography is a way to hide information inside something else. The hidden information might be a secret note, a message, or a whole file. It looks normal at first glance. For example, a person could write a secret message using invisible ink between the lines of a regular letter, or hide data inside a picture or video file on a computer.

The word steganography comes from Greek. It means "covered or concealed" and "writing." People have used steganography for hundreds of years. The term was first recorded in 1499 by Johannes Trithemius in his book Steganographia. The book also talked about cryptography.

One big advantage of steganography is that it hides the message and also hides the fact that a message is being sent. This is different from encryption. Encryption only hides the meaning of the message but shows that something secret is being sent. With steganography, even if someone sees the message, they might not realize there is anything hidden inside.

Today, people often use steganography with computer files. Images and videos are usually very large, so they are good for hiding information. For example, someone could change the color of every hundredth pixel in a picture slightly to represent a secret message. These changes are so small that someone who is not looking closely would not notice anything unusual.

History

People have used steganography, or secret messages, since ancient times. Around 440 BC in Greece, a writer named Herodotus told stories of clever ways to hide messages.

One story is about a man named Histiaeus. He had a servant’s head shaved and wrote a secret message on the scalp. When the hair grew back, the servant could carry the hidden note safely.

Another story is about Demaratus. He wrote a warning on a piece of wood under a wax tablet and then covered it with wax so no one could see it.

Much later, in the 1400s, a monk named Johannes Trithemius found a way to hide words inside a religious book written in Latin. He used a special sentence to secretly spell out a hidden word.

Techniques

Main article: List of steganography techniques

There are many ways to hide messages inside other things. One old way is to write with special ink that’s hard to see or to hide a message in the patterns of yarn on a traveler’s clothes.

Today, people often hide messages inside computer files like pictures or music. They do this by changing very small parts of these files. For example, they might change the color of a few pixels in a picture or adjust the sounds in a music file. These tiny changes carry the hidden message without anyone noticing. People can even hide messages in the way voices are sent over the internet.

Additional terminology

Further information: Security through obscurity

Steganography uses special words to describe its parts. The payload is the hidden data. The carrier is what hides the payload, like a picture. The channel is the type of file, such as a JPEG image. When the payload is hidden inside, it's called a package, stego file, or covert message. The encoding density tells us how much of the file was changed to hide the data.

Sometimes, files that might have hidden data are called suspects. If analysis shows a file is likely to have hidden data, it is called a candidate.

Countermeasures and detection

Finding hidden messages in objects can be hard and often needs special tools like magnifying glasses or ultraviolet light. During World War II, special paper was given to prisoners so hidden messages written with invisible ink would show up.

In computers, finding hidden messages is called steganalysis. One way to find these messages is by comparing files to see if anything looks different. Some hidden messages are easier to find, especially if they change how the file normally looks. But some ways of hiding messages make them very hard to find, even with advanced tools. The best way to stop these hidden messages is to change the data so the messages are gone, a process called Content Threat Removal.

Applications

Main article: Printer steganography

Some modern computer printers, like Hewlett-Packard and Xerox brands, use steganography. They add tiny yellow dots to each page. These dots have hidden information, like the printer's serial number and the time the page was printed.

Digital pictures are often used to hide messages online. This is because pictures have a lot of data. By slightly changing the color of each pixel, a hidden message can be stored. This makes it hard to see that a secret message is in the image.

Steganalysis

Main article: Steganalysis

Steganalysis is a way to find hidden messages that are hidden inside other files, like pictures or videos. There are different ways to do this, depending on what the person looking knows and what they want to do.

When trying to find hidden messages, someone might know the hidden file, the original file, or just the file with the hidden message. They might want to know if there is a hidden message, or they might want to stop the hidden message from being sent.