Information Hiding
and its Applications
Steganography and Watermarking
A detailed look at Steganography
   ◦ Text Steganography
   ◦ Hypertext Steganography
   ◦ Audio Steganography
   ◦ Image Steganography
   ◦ Steganography in Open System
Image Steganography Techniques
   ◦ Spatial Domain LSB Insertion
   ◦ Masking and Filtering
   ◦ DCT-based Steganography
   ◦ Wavelet-based Steganography
How to Detect Steganography
   ◦ Blind Detection
   ◦ Analytical Detection

Introduction

The earliest recordings of Steganography were by the Greek historian Herodotus in his chronicles known as "Histories" and date back to around 440 BC. Herodotus recorded two stories of Steganographic techniques during this time in Greece. The first stated that King Darius of Susa shaved the head of one of his prisoners and wrote a secret message on his scalp. When the prisoner’s hair grew back, he was sent to the Kings son in law Aristogoras in Miletus undetected. The second story also came from Herodotus, which claims that a soldier named Demeratus needed to send a message to Sparta that Xerxes intended to invade Greece. Back then, the writing medium was text written on wax-covered tablets. Demeratus removed the wax from the tablet, wrote the secret message on the underlying wood, recovered the tablet with wax to make it appear as a blank tablet and finally sent the document without being detected.

Romans used invisible inks, which were based on natural substances such as fruit juices and milk. This was accomplished by heating the hidden text, thus revealing its contents. Invisible inks have become much more advanced and are still in limited use today. During the 15th and 16th centuries, many writers wrote on Steganographic techniques such as coding techniques for text, invisible inks, and incorporating hidden messages in music.

Steganography:

Steganography or Stego as it is often referred to in the IT community, literally means, "Covered writing" which is derived from the Greek language. Steganography is also defined as follows, "Steganography is the art and science of communicating in a way which hides the existence of the communication. In contrast to Cryptography, where the enemy is allowed to detect, intercept and modify messages without being able to violate certain security premises guaranteed by a cryptosystem, the goal of Steganography is to hide messages inside other harmless messages in a way that does not allow any enemy to even detect that there is a second message present".

In a digital world, Steganography and Cryptography are both intended to protect information from unwanted parties. Both Steganography and Cryptography are excellent means by which to accomplish this but neither technology alone is perfect and both can be broken. It is for this reason that most experts would suggest using both to add multiple layers of security.
Steganography can be used in a large amount of data formats in the digital world of today. The most popular data formats used are .bmp, .doc, .gif, .jpeg, .mp3, .txt and .wav. Mainly because of their popularity on the Internet and the ease of use of the steganographic tools that use these data formats. These formats are also popular because of the relative ease by which redundant or noisy data can be removed from them and replaced with a hidden message. Steganographic technologies are a very important part of the future of Internet security and privacy on open systems such as the Internet. Steganographic research is primarily driven by the lack of strength in the cryptographic systems on their own and the desire to have complete secrecy in an open-systems environment.

Many governments have created laws that either limit the strength of cryptosystems or prohibit them completely. This has been done primarily for fear by law enforcement not to be able to gain intelligence by wiretaps, etc. This unfortunately leaves the majority of the Internet community either with relatively weak and a lot of the times breakable encryption algorithms or none at all. Civil liberties advocates fight this with the argument that “these limitations are an assault on privacy”. This is where Steganography comes in. Steganography can be used to hide important data inside another file so that only the parties intended to get the message even knows a secret message exists. To add multiple layers of security and to help subside the "crypto versus law" problems previously mentioned, it is a good practice to use Cryptography and Steganography together. As mentioned earlier, neither Cryptography nor Steganography are considered "turnkey solutions" to open systems privacy, but using both technologies together can provide a very acceptable amount of privacy for anyone connecting to and communicating over these system.

Watermarking:

Watermarking can be described as a method for embedding information into another signal. In case of digital images, the embedded information can be either visible or hidden from the user. Typical usage scenarios for watermarking are copyright protection and data authentication. The distinction between steganography and watermarking is not always clear. Basically, in case of watermarking the additional information is used to protect the original image (in case of copyright management), whereas in the Steganography the image is used to protect the additional information (secret messages). By definition, the visible watermarks are not included in Steganography.

During the past ten years, digital watermarking has attracted the attention of numerous researchers. As a result, hundreds of studies have been published concerning the different methods for watermarking. The information embedded as a watermark can be almost anything. It can be a bit string representing copyright message, serial number, plain text, etc. However, sometimes it can be more useful to embed a visual watermark (corporate logo) instead of a bit string as a watermark. One of the first techniques for watermarking is the least-significant bit modification. It is based on the substitution of LSB plane of the cover image with the given watermark.

It is desirable that the watermark can not be removed from the cover image. However, several intentional and unintentional operations with the watermarked image may provide possibility for disabling the watermark. Commonly, these operations (especially the intentional ones) are referred as attacks against watermarks.

An interesting application for digital watermarking is image authentication. With the help of advanced image editing software, digital images can be manipulated maliciously. In image authentication, a specific watermark is inserted into image, so that all attempts to manipulate the content of the image will alter the watermark also.

Reference:
• A Detailed Look at Steganographic Techniques and their Use in an Open-Systems Environment, by Bret Dunbar, January 2002.