# An Efficient Steganographic Protocol for WebP Files
## 4 Discussion
In this way, a single image may contain more than one value, which gives more effective transfer than in JPEG images. Therefore, we may conclude that the WebP format offers more potential techniques of data hiding than JPEG. Considering the amount of data possible to conceal, some methods offer fixed capacity, while others depend on some parameters or traits of the carrier. With simple injection, we are able to append data of any size to the cover image in exchange for increasing file size. This is because the data are added at the end of the file where they are not constrained by any marker or rules imposed by standard. Metadata and chunk injection techniques are limited by RIFF format specification, as chunk size is stored on four bytes. The maximum possible chunk size is then 4,294,967,295 bits, which is about 536.87 MB. However, the real capacity is a bit less, as image data and headers also take up space. For high-resolution images, a few megabytes may be used and the available space should be reduced by this amount. The capacity of thumbnail information hiding depends on the selected embedding algorithm. For F5, it is about 13% of the image size [6]. For other methods, the capacity may be equal to a number of non-zero coefficients, so it may vary for plain and detailed images. Data-based techniques are normally very sensitive to carrier contents. For example, the maximum possible length of a message hidden behind transparent pixels depends on their number. Similarly, in the prediction mode algorithm, the more blocks, the more bits we are able to embed. Luma blocks are \(16\times 16\) px and chroma blocks \(8\times 8\) px, which are later divided into subblocks of size \(4\times 4\). Therefore, the number of blocks is connected to image size, which has its own restriction of 16,383 pixels per side. In color indexing transform, each pixel may encode a single bit; therefore, the maximum capacity is equal to the number of pixels. The last approach uses a database of potential carriers that are stored in multiple variants (or recomputed every time). Because each container may encode one character, in a single conversation, we are able to transmit the number of characters equal to the image set size at maximum. With different encoding, this value may vary. For example, the parties may decide to diminish the amount of transferred data to one bit per image in order to arouse less suspicion. The summary is presented in Table 1.  
For format-based methods, execution times are almost invulnerable to image contents. This is because messages are hidden in specific fields or after the image data. In simple injection, embedding time results simply from disc operations; on an SSD drive, such times are counted in microseconds. Recovering is about two times larger, as it needs to read the stego file, finding the end of the original WebP image and storing the remaining \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & **Method** & **Capacity** \\ \hline \multirow{3}{*}{Format-based} & Simple injection & unlimited \\  & Thumbnail & algorithm-dependent \\  & Metadata & up to a few hundred MB \\  & Chunk injection & up to a few hundred MB \\ \hline \multirow{3}{*}{Data-based} & Transparency & 3\(\times\) transparent pixel number bytes \\  & Prediction modes & number of block bits \\ \cline{1-1}  & Color indexing transform & number of pixel bits \\ \cline{1-1}  & Container selection & image set size characters \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of capacities of presented methods. part on the disc. Thumbnail and metadata hiding are similar in terms of time after the thumbnail is ready. Then, both methods place the secret in a specific chunk and add it to the carrier. Both embedding and extracting are counted in microseconds. In the thumbnail technique, additional time is needed for thumbnail creation, which is dependent on the steganographic algorithm. For data-based methods, times depend strongly on the carrier.