Hide information in images using Steg

Even before the revelations by Edward Snowden, many users worried about how to effectively protect their private databases stored locally or in the cloud. Since minor weaknesses became apparent in the well-known TrueCrypt encryption software in a large-scale audit [1], many people have been searching for alternatives. Steganography is a little-known technology that not only protects data from prying eyes but also obfuscates its very existence.

Basics

The technique of steganography relies on hiding sets of data in a carrier medium. An image or audio file in which the message is embedded in the form of a text file is usually used for this purpose. With steganographically modified carrier media (unlike with encrypted files or encrypted file containers), it isn't usually possible for outsiders to detect that it contains a message – this significantly reduces the risk of a possible attack.

Some steganographic tools offer an extra option for particularly cautious individuals; you can, for example, encrypt the data before embedding it and add password protection. Then, the embedded content cannot be read by the attacker even after a successful attack on the data carrier and a second cryptographic attack would be needed.

Another possible use of steganography software is to provide a data carrier with an invisible digital watermark to, for example, identify copyrighted works. This means the origin of a piece of work can be proven beyond doubt in case of a dispute. However, steganographic watermarks are lost as soon as you edit the target file after embedding the watermark – by converting to a different format, for example.

Technology

Image files with high color depth are usually used as steganographic carrier media because they are particularly good at embedding messages without a detectable change in the image. It does not matter whether the data carrier is a medium with lossless or lossy compression. Photos taken with a digital camera and images that you digitize with a scanner are equally suitable for steganographic purposes.

Popular steganographic programs embed the message in a pixel's least significant bit of the color value as this is where modifications are least noticeable. The software automatically determines suitable pixels for avoiding striking changes with areas of the same color. Additionally, the algorithm disperses the embedded messages across the whole image area to prevent discovery by attacks using statistical methods.

The limited capacity of the carrier medium is the biggest disadvantage of steganographic encryption: Several megabytes of information can only be hidden in very large image files with high color depth. Image distortions would otherwise be visible to the naked eye with smaller images.