CF 106 Steganography

Transform-Domain Techniques

A transformed space is generated when a file is compressed at the time of transmission. This transformed space is used to hide data. The three transform techniques used when embedding a message are: discrete cosine transform (DCT), discrete Fourier transform (DFT), and discrete wavelet transform (DWT). These techniques embed the secret data in the cover at the time of the transmission process.

Cryptography

Cryptography is the art of writing text or data in a secret code. It involves encrypting plaintext data into an unreadable format called a ciphertext. This encryption process is based on mathematical algorithms. These algorithms use a secret key for secure encryption (Figure 1-10). The following are three types of cryptographic schemes used: • Secret-key (or symmetric) cryptography • Public-key (or asymmetric) cryptography • Hash function

Graphics Interchange Format (GIF)

GIF files are compressed image files that make use of a compression algorithm developed by CompuServe. GIF files are based on a palette of 256 colors. They are mainly used for small icons and animated images since they do not have the color ranges needed for high-quality photos.

Direct sequence

In direct-sequence encoding, the information is divided into small parts that are allocated to the frequency channel of the spectrum. The data signal is combined during transmission with a higher data-rate bit sequence that divides the data based on the predetermined spread ratio. The redundant nature of the data-rate bit sequence code is useful to the signal-resist interference, allowing the original data to be recovered.

technical steganography,

In technical steganography, physical or chemical methods are used to hide the existence of a message. Technical steganography can include the following methods

Known-stego attack:

In this attack the steganography algorithm is known, and the original object and the stego-objects are available.

Visual semagrams

In this technique a drawing, painting, letter, music, or any other symbol is used to hide the information. For example, the position of items on a desk or Web site may be used to hide some kind of message.

Text semagrams

In this technique, a message is hidden by changing the appearance of the carrier text. Text can be changed by modifying the font size, using extra spaces between words, or by using different flourishes in letters or handwritten text.

Echo Data Hiding

In this technique, an echo is introduced into the original signal. Three properties of this echo can then be varied to hide data: • Initial amplitude • Decay rate • Offset

Jargon codes

In this type of open code, a certain language is used that can only be understood by a particular group of people while remaining meaningless to others. A jargon message is similar to a substitution cipher in many respects, but rather than replacing individual letters the words themselves are changed.

Grill ciphers

It is possible to encrypt plaintext by writing it onto a sheet of paper through a separate pierced sheet of paper or cardboard. When an identical pierced sheet is placed on the message, the original text can be read. The grill system is difficult to crack and decipher, as only the person with the grill (sheet of paper) can decipher the hidden message.

Joint Photographic Experts Group (JPEG)

JPEG files are the proper format for photo images that need to be small in size. JPEG files are compressed by 90%, or to one-tenth, of the size of the data.

Masking and Filtering

Masking and filtering techniques are commonly used on 24-bit and grayscale images. Grayscale images that hide information are similar to watermarks on paper and are sometimes used as digital versions. Masking images entails changing the luminescence of the masked area.

Algorithms and Transformation

Mathematical functions can be used to hide data that are in compression algorithms. In this technique, the data are embedded in the cover image by changing the coefficients of an image, (e.g., discrete cosine transform coefficients).

Null ciphers

Null ciphers hide the message within a large amount of useless data. The original data may be mixed with the unused data in any order—e.g., diagonally, vertically, or in reverse order— allowing only the person who knows the order to understand it.

Text files

Open-space, Syntactic, Semantic

Perceptual Masking

Perceptual masking is the interference of one perceptual stimulus with another, resulting in a decrease in perceptual effectiveness (Figure 1-6). This type of steganography makes one signal hard to identify due to the presence of another signal.

File Generation

Rather than selecting a cover to hide a message, this technique generates a new cover file solely for the purpose of hiding data. A picture is created that has a hidden message in it. In the modern form of file generation, a spam-mimic program is used. Spam mimic embeds the secret message into a spam message that can be e-mailed to any destination.

End-of-line spacing

Secret data is placed at the end of a line in the form of spaces. This allows more room to insert a message but can create problems when the program automatically removes extra spaces or the document is printed as hard copy.

Semagrams

Semagrams hide information through the use of signs or symbols. Objects or symbols can be embedded in data to send messages. Semagrams can be classified into the following types:

Spread-Spectrum Encoding

Spread-spectrum encoding encodes a small-band signal into a wide-band cover. The encoder modulates a smallband signal over a carrier.

Stego-forensics

Stego-forensics is an area of forensic science dealing with steganography techniques to investigate a source or cause of a crime. Different methods of steganalysis can be used to unearth secret communications between antisocial elements and criminals.

Tagged Image File Format (TIFF)

The TIFF file format was designed to minimize the problems with mixed file formats. This file format did not evolve from a de facto standard. It was made as the standard image file format for image file exchange.

Chosen-stego attack:

The chosen-stego attack takes place when the forensic investigator generates a stegomedium from the message using a special tool.

Known-cover attack

The known-cover attack is used with the presence of both a stego-medium and a cover medium. The attacker can compare both media and detect the format change.

Known-message attack:

The known-message attack presumes that the message and the stego-medium are present and the technique by which the message was embedded can be determined.

Embedded message

The original secret message to be hidden behind the cover medium

Stego-only attack

The stego-only attack takes place when only the stego-medium is used to carry out the attack. The only way to avoid this attack is by detecting and extracting the embedded message.

Digital file types

The three digital file types are text files, audio files, and video files.

Invisible inks:

These are colorless liquids that need heating and lighting in order to be read. For example, if onion juice and milk are used to write a message, the writing cannot be seen unless heat is applied, which makes the ink turn brown.

Intersentence spacing

This method encodes a binary message by inserting one or two spaces after every terminating character. This method is inefficient since it requires more space for a small message, and the white spaces can be easily spotted.

Syntactic Steganography

This method manipulates punctuation to hide messages. Look at the following example: • Laptop, iPod, USB • Laptop iPod USB The punctuation marks are missing in the second phrase. These punctuation marks can be used to hide the message.

Semantic Steganography

This method of data hiding involves changing the words themselves. Semantic steganography assigns two synonyms primary and secondary values. When decoded, the primary value is read as 1 and the secondary as 0.

Microdots:

This method shrinks a page-sized photograph to 1 mm in diameter. The photograph is reduced with the help of a reverse microscope.

Statistical Method

This method uses a one-bit steganographic scheme. It embeds one bit of information in a digital carrier, creating a statistical change. A statistical change in the cover is indicated as a 1. A 0 indicates that a bit was left unchanged (Figure 1-7). The work is based on the receiver's ability to differentiate between modified and unmodified covers.

Interword spacing

This method uses right justification, by which the justification spaces can be adjusted to allow binary encoding. A single space between words is 0, and two spaces is 1.

Open-Space Steganography

This method uses white space on the printed page. Open-space methods can be categorized in the following three ways:

Distortion Technique

This technique creates a change in the cover object in order to hide the information. An encoder performs a sequence of modifications to the cover that corresponds to a secret message. The secret message is recovered by comparing the distorted cover with the original (Figure 1-8). The decoder in this technique needs access to the original cover file.

Covered ciphers

This technique hides the message in a carrier medium that is visible to everyone. Any person who knows how the message is hidden can extract this type of message. Covered ciphers can be both null and grill ciphers.

Frequency hopping

This technique is used to divide the bandwidth's spectrum into many possible broadcast frequencies. Frequency hopping devices require less power and are cheaper, but are less reliable when compared to direct sequence spectrum systems.

Least-Significant-Bit (LSB) Insertion

Using the LSB insertion method, the binary representation of the hidden data can be used to overwrite the LSB of each byte inside the image. If the image properties indicate that the image is 24-bit color, the net change is minimal and can be indiscernible to the human eye.

Digital watermark

a digital stamp embedded into a digital signal

Linguistic steganography

hides messages in the carrier in several ways. The two main techniques of linguistic steganography involve the use of semagrams and open codes.

Stego-medium

the combined cover medium and embedded message used in steganography

Stegosystem

the mechanism used in performing steganography

Cover medium

the medium used to hide a message with steganography

Steganography

the practice of embedding hidden messages within a carrier medium

Stego-key

the secret key used to encrypt and decrypt messages hidden by steganography

The following steganography techniques are used to hide a message in an image file:

• Least-significant-bit (LSB) insertion • Masking and filtering • Algorithms and transformation

The following steps are involved in hiding the data:

• The steganography tool makes a copy of an image palette with the help of the red, green, and blue (RGB) model. • Each pixel of the eight-bit binary number LSB is substituted with one bit of the hidden message. • A new RGB color in the copied palette is produced. • With the new RGB color, the pixel is changed to an eight-bit binary number. Look at the following example: 01001101 00101110 10101110 10001010 10101111 10100010 00101011 10101011