Secret Communication Techniques

This chapter introduces the reader to cryptography, steganography, watermarking, and quality parameters of image steganography techniques. Cryptography is a technique for secret communication. Steganography is a technique for secret and unnoticeable communication. The watermarking techniques hide watermarks inside the digital media. There are four types of steganography techniques: (1) image steganography, (2) audio steganography, (3) video steganography, and (4) text steganography. The quality of image steganographic algorithms can be measured by three parameters like (1) hiding capacity, (2) distortion measure, and (3) security check.

Steganography Using LSB Substitution and Pixel Value Differencing

This chapter describes four techniques that use both the principles of LSB substitution and pixel value differencing (PVD). These techniques use 1×3, 2×2, 2×3, and 3×3 size pixel blocks. In a block, LSB substitution is applied on one of the pixels, which is designated as the central pixel. Using the new value of this central pixel, pixel differences are computed with all its neighboring pixels. Based on these pixel value differences, embedding capacity is decided, and embedding is performed by addition and subtraction operations. The experimental results reveal that when the block size increases, the tolerance to RS analysis and pixel difference histogram (PDH) analysis also increases.

Steganography Using Substitution Principle

In this chapter, the author describes the various substitution-based image steganography techniques. Basically, there are four categories of substitution techniques: (1) least significant bit (LSB) substitution, (2) LSB array-based substitution, (3) group of bits substitution (GBS), and (4) adaptive LSB substitution. The LSB substitution-based techniques are very much insecure (detectable by RS analysis), so LSB array, GBS, and adaptive LSB substitution techniques are proposed in literature. If substitution principle is used in a steganography technique, then it must be evaluated by RS analysis.

PVD Steganography Based on Correlation and Maximum Pixel Value Difference

In this chapter, the author describes the pixel value correlation-based steganography techniques and maximum pixel value difference-based steganography techniques. The pixel value-based correlation steganography techniques utilize the relationship among the pixel values of a block, whereas the maximum pixel value difference steganography techniques utilize the largest pixel value difference among different pairs of pixels in the block. Although the PSNR values of these techniques are better as compared to the traditional PVD schemes, the hiding capacity is not improved.

Steganography Techniques Based on Modulus Function and PVD Against PDH Analysis and FOBP

This chapter proposes two improved steganography techniques by addressing two problems in the existing literature. The first proposed technique is modulus function-based steganography and it addresses pixel difference histogram (PDH) analysis. The modulus function is used to calculate an evaluation function and based on the value of the evaluation function embedding decision is taken. There are two variants of this technique: (1) modulus 9 steganography and (2) modulus 16 steganography. In modulus 9 steganography, the embedding capacity in a pair of pixels is 3 bits, and in modulus 16 steganography the embedding capacity in a pair of pixels is 4 bits. Both the variants possess higher PSNR values. The experimental results prove that the PDH analysis cannot detect this technique. The second proposed technique is based on pixel value differencing with modified least significant bit (MLSB) substitution and it addresses fall off boundary problem (FOBP). This technique operates on 2×2 pixel blocks. In one of the pixels of a block data hiding is performed using MLSB substitution. Based on the new value of this pixel, three difference values with three neighboring pixels are calculated. Using these difference values, PVD approach is applied. Experimental results prove that the PDH analysis and RS analysis is unable to detect this proposed technique. The recorded values of bit rate and peak signal-to-noise ratio are also satisfactory.

Adaptive LSB Substitution and Combination of LSB Substitution, PVD, and EMD

This chapter describes two types of steganography techniques: (1) adaptive LSB substitution and (2) combination of LSB substitution, PVD, and exploiting modification directions (EMD). The adaptive LSB substitution technique embeds a variable number of bits in different pixel blocks depending on the smoothness of the block. This improves the security and reduces the distortion. In the second technique, the LSB substitution, PVD, and exploiting modification directions (EMD) principles are judiciously combined in pixel blocks to get higher embedding capacity, lesser distortion, and improved security. These hybrid techniques survive from both RS analysis and pixel difference histogram (PDH) analysis.

Pixel Value Differencing Steganography

In this chapter, the author describes the different categories of pixel value differencing (PVD) techniques and their performances. The main goal in PVD technique is to find the pixel value difference. If it is large, hide the larger number of bits; otherwise, hide the lesser number of bits. The traditional PVD techniques operates on 2, 4, and 8 pixel blocks to calculate the pixel value difference and then take the embedding decision. The traditional PVD techniques use a range table to decide the embedding capacity in a block. The adaptive PVD techniques do not use any range table and calculate the embedding capacity dynamically for every block based on the pixel values of that block. Most of the traditional PVD techniques are attacked by pixel difference histogram (PDH) analysis, but adaptive PVD techniques are tolerant to PDH analysis.