The main methods of protection of multimedia information are algorithms based on steganography. Steganography is the science that ensures the exchange of information in such a way that it hides the very existence of secret communication. It does not replace cryptography (encryption), but complements it with another level of security. During data processing by steganographic methods the information is hidden in other objects (files, disks, etc.) so that aт unauthorized person does not guess the existence of a hidden secret message. In addition to the hidden messages, steganography is one of the most promising areas of authentication and labeling of the author's products. Often the information about the place and date of a product creation, data about the author, license number, serial number, etc are used as hidden information. This information is usually implemented in the visual and audio works, as well as in protected software products.

In modern computer steganography there are two main types of files: a message - the file that is designed to be hidden, and a container - the file that can be used to hide the message. Containers are of two types. The container-original (or "empty" container) is a container, which contains no hidden information. The container-result (or "full" container) is a container that contains hidden information. A key is a secret element, which determines the procedure (algorithm) of recording of a message into the container.

This article analyzes in more details methods of embedding hidden information in image files, as well as evaluating the sustainability of stegosystems. Using as containers image files, you can build not only textual information but also images and other files.

The only condition is that the hidden image should not exceed the size of the image repository. To achieve this, each program uses its own technology, but they all are brought down to the replacement of certain pixels in the image.

A digital image is a matrix of pixels. As known, the pixel is a single element of the image. It has a fixed dimension of the binary representation. For example, the pixels in grayscale images are encoded in 8 bits (the value of brightness varies from 0 to 255).

Low significant bit (LSB) of an image carries the least amount of information. We know that people are not usually able to notice changes in the lower bits, and especially in the lowest. In fact, the value of a low bit is random, not carrying information to be identified by human eyes. Therefore, it can be used to embed information. Thus, for grayscale images the amount of embedded data may be 1 / 8 of the container volume.

The actual images allow to send large amounts of information. Thus, for the graphic image of 1024x768 it is possible to send 294912 bytes in the closed type, using the method of least significant bits, LSB.

For example, with three colors RGB each pixel is described by three bytes, i.e. 24-bits.

For the case of the introduction of secret information in the form of symbol A 01000001 in three consecutive pixels, each of which is described by three bytes:

                                                                (00100111, 11101001, 11001000)

                                                                (00100111, 11001000, 11101001)

                                                                (11001000, 01000111, 11101001)

only a part of low bits values of corrected values of bytes will be modified. As a result, we get

                                                                (00100110, 11101001, 11001000)

                                                                (00100110, 11001000, 11101000)

                                                                (11001000, 01000111, 11101001)

Unfortunately, not all image formats preserve the value of lower level with different kinds of transformations (compression, unpacking). The fact of coding is easy to detect, because the statistical properties of lower bits will be determined by encrypted text and different from the random equally probable values. Most often, the secret message or CEH is originally encrypted using a cryptographic method based on a secret key.

In color graphics, you can replace not only the lowest bit, but two or three low ones. These changes are hardly distinguishable by human eyes. As a container it is desirable to choose the scanned image. Only in the quantization noise random filling of low bits which disguise pieces of stegosomessage are present.

You should avoid photos with large areas of very bright and black. In these pictures big fields in the source file have color components 255 and 0, so stegobytes will stand out when viewing a file in the code in 254 and 1. 73 Using steganographic systems, keep in mind that the more information is embedded in the file container, the lower the reliability of the system is.

Currently, the most popular format of images is RGB format, according to which information about the color of the point is stored in the form of three color components. Most steganographic methods work with images in this presentation. For methods that use the allocation of spatial frequencies, when using as containers of colored images it is previously required to transfer in the sphere of luminance and colour signals:

To record the information it is recommended to use only the luminance component, as it best describes the processed image, and less prone to distortion. Colour components are encouraged to be used in the future for a possible correction in order to counter the new methods of stegoanalyses. At the moment they are only used to compensate for color distortions of individual points in the reverse transition to the submission of RGB. The provision for recording information of points in areas with high spatial frequency by using spatial filters is based on the matrix:

These masks are most suitable for specific codes, which helped solve a number of problems associated with the high level of secrecy, resilience and capacity. Despite the fact that the distortion of the individual points of reaching tens of percent, they remain invisible because of the high frequency of the image, where a clear prediction of the values of amplitude and frequency components of spatial function of the image is impossible.

There are a lot of software products that are used for the purpose of steganography and realizing, as a rule, means of the introduction of secret data in graphic, audio and video files. Many of them are free or conditionally free. The most popular ones are EZStego, DiSi-Steganograph, Steganos, etc. The use of majority of them is reduced to pressing a few buttons in dialog boxes - just select the message file that you want to hide, then file - the receiver of data, which has to hide data (its capacity should be sufficient to store the embedded data) and click OK.

Literature

1. https://www.pgpru.com/biblioteka/steganografija

2. Andreas W., Andreas P., “Attacks on Steganographic Systems. Breaking the Steganographic Utilities EzStego, Jsteg, Steganos, and S-Tools – and Some Lessons Learned”, Proceedings of the Workshop on Information Hiding, 1999.

3. Быков Р.Е., Фрайер Р., Иванов К.В., Манцветов А.А. Цифровое преобразование изображений: Учебное пособие для вузов. – М., 2003. – 228с.

© Ларионова К.Е. 2009