Steganography

Steganography is the practice of hiding information inside another message or object in a way that makes it look like something else. This hidden information might be a secret note, a message, or even a whole file that seems completely 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, combining words that mean "covered or concealed" and "writing." People have used steganography for hundreds of years, and the term was first recorded in 1499 by Johannes Trithemius in his book Steganographia, which also talked about cryptography.

One big advantage of steganography is that it hides not just the message, but also the fact that a message is being sent at all. This is different from encryption, which only hides the meaning of the message but still makes it clear that something secret is being sent. With steganography, even if someone sees the message, they might never realize there is anything hidden inside.

Today, people often use steganography with computer files. Because images and videos are usually very large, they are great 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

The first known uses of steganography date back to 440 BC in Greece. The historian Herodotus wrote about two clever ways people hid messages. One story tells of Histiaeus who had a servant’s head shaved to write a secret message on his scalp. After the hair grew back, the servant traveled to deliver the hidden note. Another example is Demaratus, who wrote a warning on the wood underneath a wax tablet before covering it with wax.

Later, in the 15th century, a monk named Johannes Trithemius created a way to hide words inside a religious Latin text. One of his examples used the phrase "Auctor sapientissimus conseruans angelica deferat nobis charitas potentissimi creatoris" to secretly spell out the word VICIPEDIA.

Techniques

Main article: List of steganography techniques

Many ways have been found to hide messages inside other things. One old way is to use things you can see or touch, like writing with special ink that’s hard to spot or hiding a message in the patterns of yarn worn by a traveler. In modern times, messages are often hidden inside computer files such as pictures or music.

With computers, people hide messages by changing tiny parts of digital files. For example, they might adjust the colors of a few pixels in a picture or change the way sounds are made in a music file. These small changes carry the hidden message without anyone noticing. Even in talking over the internet, people can hide messages in the way voices are sent and received.

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

Detecting hidden messages in physical objects can be difficult and often needs special tools like magnification or ultraviolet light. During World War II, special paper was given to prisoners so any hidden messages written with invisible ink would become visible.

In computers, finding hidden messages is called steganalysis. One way to spot these messages is by comparing files to known originals to see if anything has changed. There are different methods to find hidden messages, such as looking for patterns or changes in data. Some hidden messages are easier to find, especially if they change the statistics of the file they are hidden in. However, some techniques make it very hard to detect hidden messages, even with advanced tools. These hidden messages can be used for cyber attacks, known as Stegware. The best way to stop these attacks is to change the data so any hidden messages are destroyed, a process called Content Threat Removal.

Applications

Main article: Printer steganography

Some modern computer printers, like Hewlett-Packard and Xerox brands, use steganography by adding tiny yellow dots to each page. These dots contain hidden information, such as the printer's serial number and the date and time the page was printed.

Digital pictures are often used to hide messages online because they contain a lot of data. For example, by slightly changing the color of each pixel in a way that is not noticeable to the human eye, a hidden message can be stored. This makes it harder for someone to tell that a secret message is present in the image.

Steganalysis

Main article: Steganalysis

Steganalysis is a way to find hidden messages that are concealed inside other files, like images or videos. There are different methods to do this, depending on what information the person searching has and what they want to achieve.

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