# A Novel and Robust Hybrid Blockchain and Steganography Scheme
## 3 Methodology and Proposed Method
### Steganography: An Overview  
Steganography is studied in four main domains. These are the spatial domain, the transform domain, the spread spectrum domain, and the model-based domain. The data bits to be hidden in the spatial domain are directly hidden in the cover image. The Least Significant Bit (LSB) [30] method is a widely used and known algorithm in this field. The use of the LSB algorithm is simple, fast, and it is very difficult to detect visually without statistical methods. However, in case of a change as a result of any effect on the multimedia where the data is hidden, it becomes impossible to access the hidden data. Pixel Value Differencing (PVD) and Binary Pattern Complexity (BPC) are a few other methods used in the spatial domain. The data to be hidden in the transform domain is hidden in the cover multimedia using transposition. The most familiar methods are Discrete Cosine Transform (DCT) [31; 32], Discrete Haar Wavelet Transform (DWT) [33], and Discrete Fourier Transform (DFT). Considering that the cover used in DCT will be an image, it divides the image into \(8\times 8\) blocks. In these blocks, sensitive areas to hide data are determined, and then steganography takes place. Considering that the cover used in DWT will be an image, the cover image is divided into four sub-bands. These sub-bands are high-high (HH), high-low (HL), low-high (LH), and low-low (LL). Data hiding is preferred in the LL band. Regardless of the data size hidden using the DWT algorithm, when you divide the cover multimedia into sub-bands and then combine them, the multimedia you get will not have the same structure. The DWT algorithm causes permanent corruption on the cover multimedia, but this situation does not affect the stego-data, which is hidden in the LL band. In DFT, discrete-time signals are converted to discrete frequencies. However, discrete-time is used and converted to continuous frequency. DFT transforms a vector list of instances of a function into a finite list of coefficients ordered by frequency. Discrete Fourier transform series is a high-speed algorithm. As a result of the application of DFT on computers, fast Fourier transform is also called FFT. Spread spectrum image steganography is a process of storing an encrypted message as a Gaussian noise in a cover image. This method relies on the fact that low noise power levels cause little image degradation and therefore it is undetectable by the human eye. Even if there is some data loss in the bands, it is possible to reach the secret data by using the data left behind. As long as all cover multimedia is not removed, access to secret data can be achieved. For this reason, this technique is preferred in military applications. In a model-based domain, cover multimedia is divided into two parts; only the second of these parts is for hiding data. While performing steganography, the statistical properties of cover multimedia are not changed. As the size of the data hidden by this method increases, the detection rate using various stego-analysis methods is much lower than in the transform domain. For this reason, it is preferred in steganography studies where large amounts of data are hidden. However, data hiding and extracting from stego-multimedia in this domain take much more time than the examples seen in other domains [34]. There are requirements that steganography must meet at a successful rate in order to be considered: imperceptibility (undetectability), security, payload capacity, robustness. Imperceptibility is one of the vital requirements in steganography. Imperceptibility is the inability to detect secret data hidden in cover multimedia using various steganography techniques by statistical or visual analysis. After applying steganography, the smaller the difference between cover multimedia and stego-multimedia, the higher the imperceptibility. Histogram, peak signal-noise ratio, mean squared error, structural similarity index measure, and similar quality metrics are all used to measure imperceptibility.