Comparative results for the stegoimage “Mandrill” with the secret image “Lena”
Abstract
Two of the main problems with steganographic algorithms are insertion capability and minimization of distortion in the digital files where the hidden information is the information is inserted to hiding Digital filters are generally used as noise detectors, and they also suppress information outside the original information contained in the file. There are different types of filtering, one in the spatial domain and the other in the frequency domain or sometimes a combination of both domains to propose adaptive filters. One of the filters with greater application is the discrete wavelet transform (DWT) because it is easy to implement and has low computational complexity. The DWT computationally implemented in an image can be represented as a quadrature mirror filter, separating the frequency components: so highhigh, highlow, lowhigh and lowlow levels obtain different resolutions.
Keywords
 Steganographic algorithm
 digital filters
 frequencial domain
 spatial domain
 discrete wavelet transform
1. Introduction
Hiding information concerns the process of integrating information or data elements into music, video, and images [1]. The concept of information hiding had also been proposed to solve problems related to intellectual property protection that depends directly on a team of background work for its realization, and its distribution must be controlled, or even protection of personal information with a high degree of confidentiality such as agreements, contracts, or other information that should be secured. To send information to a file imperceptibly seems totally innocent, at first, and such a file is known as a carrier file or host file (known as a host image or cover image).These files are carriers of embedded information; they can be any digital files such as audio files, video files or image files. For intellectual property protection and for secure delivery of information on unprotected media, various techniques have been implemented that have given rise to the emergence of the science of encryption, watermarking, and steganography. For the first technique, the message will not make any sense unless acquired by the intended recipient or to whom the message is addressed, as this will be the interpretation of each of the written symbols within the message. For watermarking, this is related to protecting the authenticity of media messages exposed, susceptible to cloning or replication. With watermarks, its authenticity is guaranteed. Finally, we have steganography, which is the technique used to hide information in a seemingly innocent means to be sent over an unsecured medium or channel. A common example is the wireless medium, which is fully exposed and for anyone with enough malice to access information, it can be done without any problem. For the goals of steganography to be met, there are three considerations: (i) high capacity for integration, (ii) high quality of stegoimage and (iii) full recovery of the inserted information. The images with the inserted information are called stegoimage; the information must be inserted in areas that are statistically and perceptually not easy to locate on demand and stegotools such as analyzers. The stegoimage should be, in theory and in practice, identical to the host image. The embedded information in the stegoimage can only be removed by the receiver that has the primary key, otherwise, the information cannot be extracted. The stegoimage is then sent to the intended recipient. The information contained in the host image is just a distraction to the receiver, so this is not so much of an interest in its full recovery, but the host image must have the minimum quality because any edge, contour,, color or misplaced pixels can cause some suspicion and is susceptible to the extraction of the hidden information without authorization from the transmitter. Most importantly, the hidden information must be fully recoverable. In today’s advanced and modern world, steganography is vital because it is a support tool to copyright protection, for which the authentication processes allow distribution and legal use of different materials. A steganographic technique is usually evaluated in terms of the visual quality and the embedding capacity; in other words, an ideal steganographic scheme should have a large embedding capacity and excellent stegoobject visual quality.
One of the main objectives of steganography is not sacrifice image quality as the carrier to be inserted to this data. And likewise keeping track information retrieved data. Because any disturbance visual may cause some reason for inspection of its content through histograms or specialized software.
The more reasonable way to deal with this tradeoff situation is probably to strike a balance between the two [2, 3, 4].
The techniques implemented in steganography have evolved as needs related to security level and insertion capability have also increased, due to the growing trend in the digital distribution of files over public networks. There are several proposed techniques applied to steganography, including the space where the most representative method algorithm is the modification of the least significant bit (LSB) of the pixel, and from this algorithm, optimizations have emerged such as the LSBO, among others [5].
Subsequently, frequency techniques for image processing such as the discrete Fourier transform (DFT), discrete cosine transform of (DCT), and the discrete wavelet transform (DWT) have also emerged.
More recently, adaptive methods, take the qualities of the spatial domain and the frequency domain. These latest adjustments have yielded good results; however, few authors consider the three criteria for steganographic algorithms. In adaptive methods, statistical variations are considered image through variance, standard deviation, covariance, etc.
These statistics variations detected in images in conjunction with filtering images using the DWT techniques are useful and important to execute noise detectors within the image. These noisy areas are imperceptible to the human eye, so the eye does not see that some information have been inserted outside the original image. The human eye is less sensitive to detect certain imperfections contained in the processed images, these characteristics are due to the textures contained in the images that are shown as changes in intensity. These abrupt changes in intensity are contained in the high frequency band, which are obtained from the image processing by DWT [6].
In applications where the frequency domain is involved, depending on the nature of the image used as host, this image can be altered significantly when the digital filters applied act as noise detectors. Judging by whether the human vision sensitivity is considered in the design of the embedding algorithm, we can categorize the schemes into three types: (1) high embedding capacity schemes with acceptable image quality [24], (2) high image quality schemes with a moderate embedding capacity [24], and (3) high embedding efficiency schemes with a slight distortion [7,8]. This chapter explains how to reduce the effects of the filters on images using the scale factor proposed in this research. This scale factor will be working in the wavelet domain. Applying the scaling factor, the energy generated by the host image is preserved to approximate the original image, eliminating any visual disturbance. This chapter is divided into the following sections: Section 2. Materials and methods, Section 3. Theory and calculations, Section 4. Discussion and Section 5. Conclusions.
2. Materials and methods
To analyze structures within images of different sizes, shifting the window in Fourier transform (Gabor Transform [9]) is not suitable because it is subject to a fixed size value. Moreover, the calculation is complicated, because the DFT, obtains complex numbers, unlike the DWT [9], which operates only with real numbers. For analysis of images, it is necessary to use timefrequency windows with different times. Instead of choosing a fixed window (Gabor transform), with an analysis window g (tu) constant size, a resizable window is chosen, that is, a wavelet, a Ψ∈L^{2}function (IR), with an average equal to zero:
The DWT can decompose an image in different resolutions, each of which progressively decreases the size. This presents a certain analogy with the human eye, which draws, at each level, the information that it finds interesting. For example, consider a brick wall. If we observe it from a considerably large distance, we will see a global structure. As we approach the wall, we can look at the successive characteristic details: the divisions between bricks, each brick structure that define and detail the whole structure, losing resolution. Similarly, the DWT extracts information between successive resolutions. Consider the function checks  Ψ  = 1, and is focused on a neighborhood of t = 0. If the value of u and s in the function moves, a movement of the sampling window timefrequency is obtained:
The DWT can be written as a convolution product:
where
The DWT can find localized and structured images with an amplification procedure that progressively reduces the scale parameter. Generally, singularities and irregular structures often contain essential information about the image and refer to places that can be substituted for values, but at first glance, are not identifiable. In an image, intensity discontinuities indicate the presence of edges. It can be proven that the local regularity of a signal is characterized by the decay of the amplitude of the DWT over scales. Thus, the singularities and the edges are detected following the local maximum values of the DWT to finer scales detail. This image of singularities will become detailed as it moves [10]. To characterize the unique structures, it is necessary to quantify the local regularity of the image. For example, an image of n × m pixels generates additional images successively in blocks. All contours, large and small, are present in the original image and do not require any change of resolution to locate them. The issue is in identifying broad contours using conventional operators. It could be escalated to the operator, but what is more efficient is to scale the image because the use of an operator for large contours on a highresolution image is very complicated from the point of view of computational efficiency. Therefore, it uses coding frequency subbands. Coding performs subband decomposition of an image or a signal bandlimited components (bandpass filter), which gives a redundancyfree representation of an image; this makes it possible to reconstruct the original image without error. Give a bandlimited image
It is possible to split the image to make a uniform sampling
where
The coding of two channels per subband filtering
This results to obtaining the four subimages at the output of the processing. The image resolution applying DWT is divided into four frequency subbands called LowLow (LL), LowHigh (LH), HighLow (HL), and HighHigh (HH), see Figure 2. The names given were based on the type of filtering applied to the rows and columns; they are obtained from Eqs. 6 and 7.
Each subband is a copy of the original image but in different frequency level, which provides a certain amount of energy [8, 9] (Figure 1). To describe a DWT, it is enough to define a discrete impulse response for a lowpass filter,
where
which can be constructed as a sum, from Eq. 11, that is half copy of the image, using
and
where
Determined
and from this, we obtain the mother wavelet,
where the set of orthonormal wavelets is derived, and the scaling factor obtained for setting the pixel with the new value obtained from the lowpass and bandpass filtering from [11],
It is restricted to case basis functions obtained by means of changing the type of binary scale
The Lipschitz regularity of edge points is derived from the maximum decrease of along DWT scales. In addition, approaches the image can be reconstructed from these high module without visual degradation. In two dimensions, we try to detect contours. For detection, the problem is the presence of noise. So, if we define the boundary from turning points, they will appear across the surface, due to noise. The application of the values for Lipschitz exponents can find and ratifying the highlighted areas like the edges obtained in step filtering Lipschitz regularity defines the upper limit with noninteger exponents. Thus, the DWT is a powerful tool to measure the minimum local regularity of the tool functions. However, it is not possible to analyze the regularity of f at a particular point v that will decrease from
where
The discrete wavelet reconstruction can be computed by an inverse of the procedure of decomposition beginning at the level of resolution lower in the hierarchy. In applying the proposed steganographic algorithm to the subband LH is necessary to use a scaling factor that works with 24bit RGB color images or Luminance, Chromatic blue, Chromatic red (YcbCr) or Hue, Value, Saturation (HSV) color model [14]; this scaling factor is closely related to energy conservation applied in the theory of wavelets. However, in the RGB color images, we propose the following scaling factor,
where
The proposed steganographic method works in the wavelet domain and provides an analysis of the wavelet coefficients in different decomposition scales to estimate the simple variance field to distinguish areas where the pixels are considered noisy [14]. We propose the following criterion: If the standard deviation of the current wavelet coefficient kernel is smaller than the threshold defined for the standard deviation global, then the respective area from the host image is considered noisy and then in such area, the hidden information can be inserted. Otherwise, the pixels from this area are considered free of noise and it is not possible to insert data of the hidden image. The criterion provides good invisibility for the hidden data and edges and results to fine detail preservation of the stegoimage [14]. The standard deviation is computed using the following,
where
The threshold
where
3. Theory/Calculations
In the optimization and evaluation of algorithms in digital image processing, the peak signal to noise relation (PSNR) is the criterion most frequently used to evaluate the quality of the imagery [2],
The Normalized Color Deviation (NCD) is used for the quantification of the color perceptual error [2],
Here,
The hiding capacity (HC) dictates the number of bits inserted in the host image [2,14],
We incorporate in the proposed scheme other color spaces such as YCbCr and HSV to ensure that the visual artifacts appearing in the stegoimage are imperceptible, and the difference between the cover and stegoimage is indistinguishable by HSV by using the proposed scaling factor. To verify and quantify the results obtained in this proposed steganographic algorithm, the results obtained in conjunction with other spatial domain methods were compared with 43 LSB and the optimization of this algorithm.
Table 1 shows the performance results in terms of PSNR, MAE (Mean Absolute Error), CORR (Correlation), Q, NCD, HC, in the case of j=2^{8} values in the scaling factor by using the 320 × 320 RGB color image “Mandrill” [15] as the host image and “Lena” [15] as the hidden image. In Table 1, we can see that the best result is presented by the steganographic method proposed here by applying the scale factor adjustment in the wavelet transform, the scale factor for this example has a value of j = 2^{8}. In Table 2, we can see the results of the image recovered "Lena" where you can see that the image quality index (Q) is close to 1, indicating that the recovered image is very close to the one inserted originally.









PSNR dB  36.168  36.169  30.111  30.112  37.283 
MAE  2.9911  2.9902  9.7307  9.7315  1.2020 
CORR  99.78  99.78  99.76  99.76  99.34 
Q  0.9976  0.9976  0.9948  0.9948  0.9956 
NCD  0.00073  0.00073  0.0022  0.0022  0.00030 
HC (Kb)  51,200  38,400  51,200  38,400  76,800 









PSNR dB  36.291  36.291  30.144  30.144  35.391 
MAE  2.7479  2.7479  8.1255  8.1255  2.9409 
CORR  99.55  99.55  99.54  99.54  99.30 
Q  0.9962  0.9962  0.9943  0.9943  0.9950 
NCD  0.0020  0.0020  0.0033  0.0033  0.0019 
In Table 3, we show the results obtained from the proposed steganographic algorithm with a scaling factor, with j = 10. We note that PSNR improves by more than 1dB, and conserves the Q index on the recovered image. The PSNR value is enhanced for each of the cases in color spaces, and so it may be said that the energy distribution (inserted data) within the image is homogeneous, thus wanting to be approached by a stegoanalyzer to retain a uniform distribution histogram, removing any suspicion of being a carrier of information. In Table 3, we can see that the model of color HSV offers better quality in the stegoimage in contrast to the RGB and YCbCr models; nevertheless, the capacity of insertion is lost. It is possible to observe that the model RGB offers good results and the capacity of insertion does not sacrifice itself.




Q=0.9999  Q=0.9962 
PSNR=39.1233 dB  PSNR=37.5167 Db 
COI=99.34%  COI=99.55% 
NCD=6.0486 e4  NCD=0.0020 
MAE=1.7022  MAE=2.7714 
HC=25.65Kb  




Q=0.9888  Q=0.9962 
PSNR=30.3913Db  PSNR=36.0827 dB 
COI=98.92%  COI=99.54% 
NCD=9.5940 e4  NCD=0.0020 
MAE=2.2044  MAE=2.7948 
HC=2.18Kb  




Q=0.9999  Q=0.9962 
PSNR=41.3900 dB  PSNR=36.1233 dB 
COI=99.92%  COI=99.55% 
NCD=2.8906 e4  NCD=0.0020 
MAE=0.6401  MAE=2.7714 
HC=0.068Kb 
We also present the error images in Figure 4 presents the visual results according with Table 3.
4. Discussions
The results only apply steganographic algorithm have shown visual defects, which may cause some suspicion that this is a carrier of information added to this. However, applying the scaling factor, better visual and quantitative results also overcoming the stegoimage are obtained. Significantly, a steganographic algorithm must meet the following criteria: Total recovery of the embedded information; good quality of the cover image and the recovered image; and high insertion capability. Finally, in this work, the proposed method yielded better results, with the best result obtained using the scaling factor
5. Conclusions
The RGB, HSV and YCbCr color model images are altered in their energy contribution in each submatrix of the wavelet decomposition when the steganographic algorithm is applied. From equations 5, 6 and 14, we propose the use of the scaling factor for adjusting filtered images with DWT. This adjustment will be made to each pixel of the image to achieve the three objectives of steganographic algorithms. For steganographic applications, the digital filter helps to locate areas suitable for inserting information without it becoming visible to the human eye. This filter is generally altered in their energy contribution in each submatrix of wavelet decomposition when a steganographic algorithm is applied. It is known that the value of
Acknowledgments
This work was supported by Instituto Politécnico Nacional (IPN) and Consejo Nacional de Ciencia y Tecnología (CONACYT).
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