# An Efficient Steganographic Protocol for WebP Files
## 4 Discussion  
The methods presented in this paper are divided into two categories: the manipulation of file structure or the data. The main difference between these two approaches is that format-based algorithms do not change image contents, only the file structure. It may be seen during pixel-by-pixel comparison of images that data-based algorithms introduce some discrepancies, whereas format-based methods not. On the other hand, data manipulation does not significantly increase the file size. From all described techniques, all except one are focused on container modification, whereas the remaining one uses container selection. The original image is modified in all techniques except container selection (in which different compression levels indicate secret messages). In format-based techniques, modifications affect the file, but not the image content itself. This means that some data may be added to the container or replaced, but pixels remains identical. The second category of methods, data-based methods, introduces alterations to pixels. In color indexing, one of the components may be modified by 1. In prediction modes, alterations concern blocks. The size of the block may vary, but usually it is a square group of pixels, the values of which are changed during the embedding process. These modifications are very slight. As the WebP format is not widely discussed in the literature, we may show similarities and differences to methods working on other file types. Format-based techniques are dependent both on the standard and the software. It is best visible in simple injection. A similar technique may be applied to BMP images. Then, depending on the software used, another image may be displayed [72]. In WebP files, such behavior has not been observed. Thumbnail and metadata methods are based on the same structures (Exif and XMP) that are found in other file types. The difference is that in the thumbnail, we may choose from available algorithms of JPEG steganography, whereas in metadata, participants should invent more creative strategies. Chunk injection exploits the extendability of the standard, so other RIFF-based files may potentially use this method. Embedding data in transparent pixels is also possible in PNG images, as confirmed by the conducted tests. On the other hand, indexed images such as GIF have a limited color palette and may benefit from the color indexing method. Information hiding in prediction modes works on formats that use a specialized encoder which predicts unknown pixel values. From popular image types, only WebP implements such an algorithm. The container selection technique may be used in almost any type of file. However, WebP images are a good choice, as a single image may be compressed with various parameters, creating multiple similar versions of the same file. In this way, the container database grows significantly and users are able to transmit longer messages with fewer data sent. There are some similarities and differences between WebP and JPEG steganography. Format-based methods may be applicable for both image formats, because JPEG and WebP files allow for metadata and thumbnails. The details may vary, for example, the encoding  
Figure 7: (**Left**): pure image with its colors; (**right**): cover image with extended colors. method or embedding algorithm for the thumbnail image. On the other hand, data-based techniques are different for WebP and JPEG. While JPEG steganography usually focuses on discrete cosine transform to conceal secret information, the WebP format has more possibilities because of its frame prediction. This mechanism, derived from VP8, gives the possibility to hide a message during predictive coding. Color indexing is not available for JPEG images because this format is lossy; however, WebP supports lossless pictures as well. The same is true for transparency. The container selection approach is not necessarily connected to file format, but WebP images have more parameters that users may adjust, so their potential in steganography is greater.