Digital Steganography: The art of Hiding Files

I received a fascinating question the other day from Infopackets Reader John B. He writes:

" Dear Dennis,

I just read about a new privacy program which is able to hides files inside jpeg image files. My question is: If I downloaded a jpeg file from the Internet, can it have a virus hidden inside? The reason why I ask is because I found a jpeg image file in my temporary Internet file folder. When I tried to open it, I was told that the file .exe could not be located. I immediately deleted the jpeg and am worried that my PC is now infected. What do you think? "

My response:

What you are referring to is also known as Steganography, Digital Watermarking, or Cryptography.

Steganography is "the hiding of a secret message within an ordinary message and the extraction of it at its destination. Steganography takes cryptography a step farther by hiding an encrypted message so that no one suspects it exists. Ideally, anyone scanning your data will fail to know it contains encrypted data. In modern digital steganography, data is first encrypted ... and then inserted [into] ... a particular file format such as a JPEG image." (Source: techtarget.com)

A few comments:

1. If the file you executed had the extension .jpg or .jpeg, Windows should have attempted to load this file with an associated image viewing program (such as Explorer, Microsoft Picture Viewer, IrfanView, etc).
    
2. Steganography, as far as I understand, only hides files / information in a harmless file, called the carrier file (such as a Jpeg image). Carrier files cannot execute hidden code arbitrarily; a special utility program is required to extract and manipulate concealed data.
    
3. Quite often, hidden content is encrypted using password-protection and cannot be decrypted without the password (also known as the "key").

Having said all that: I suppose it is also possible that you could have recently downloaded and installed a steganographic image viewer which has associated itself with .jpg / .jpeg file extensions on your computer and is capable of executing potentially harmful code contained in an image file.

It's certainly possible, but not very likely.

What do you think?

This certainly is a fascinating topic, and I'd love to read comments from Infopackets Readers. Please feel free to email me your thoughts!

Update 2003/12/04: This topic has been updated. Click here to read!