# An Efficient Steganographic Protocol for WebP Files
## 4 Discussion
When hiding secret data in transparent pixels, times grow about 30% compared to image conversion to WebP. Extraction times are comparable. In the prediction mode algorithm, it turned out that focusing only on modes with appropriate identifiers shortened execution times by about 24%. This is because, in this step, the reconstruction of the image part and its score are computed. Avoiding this time-consuming code allowed for shorter compression. Recovering the message does not increase the time considerably. The container selection approach is characterized by the largest times of database generation. The reason for this is that every image needs to be prepared in multiple versions. After that, secret extraction is very fast. It only requires finding the corresponding file in its own group, which may be accomplished by comparing precomputed hashes. Considering steganalysis, an adversary may apply a few approaches. To detect format-based techniques, the adversary should analyze the carrier in search of anomalies. In our opinion, the easiest to detect is simple injection, because image size is stored in the RIFF container as a segment size. Then we may compare this value to the real image size and find discrepancies. The most difficult to detect is hiding a message in a photo's unique ID, as this field contains a random hexadecimal value which is impossible to distinguish from encrypted data. Other metadata and chunks are visible in plain text, so the adversary may read them; message security may vary depending on the selected embedding algorithm, applied encryption, resemblance of metadata to other common images etc. Similarly, thumbnail steganography detection depends purely on the algorithm used. The specific JPEG steganalysis method should be applied to check whether a thumbnail contains a secret message or not. In the container selection approach, the adversary needs to intercept a longer conversation between parties, as a single image contains no data inside. Only the analysis of multiple messages may lead to the discovery of a hidden channel based on a collection of pictures. Considering data-based methods, the adversary may try to modify the carrier or to analyze its contents. To reveal data hidden under transparent pixels, he may remove the alpha channel. The detection of messages hidden via color indexing is harder, because some images contain similar colors, and the adversary needs to decide whether such similarities are artificial or are natural phenomena present in a specific image. The interesting problem is also the detection of prediction mode steganography. It requires decoding all used modes and deciding if they contain any secret information. Format-based methods are robust to webp compression, as they are independent of pixels of the image. The one exception is simple injection, in which hidden data are ignored during re-compression and the new image does not contain a secret message. In other techniques, secret data are copied into a newly compressed image to retain metadata and all chunks. Considering data-based methods, some of them may be affected by compression. In the prediction mode technique, applying compression changes final pixel values, therefore leading to destruction of hidden data. Color indexing is only used on images with lossless compression, which means that these pictures are robust. The transparency method described in this paper was also used on lossless images. Finally, the container selection approach is based on the compression, which means that when the adversary applies compression to images in the transmission, he would be able to modify the transmitted message. When we compare the results of the study in the context of the used software, we may draw some conclusions about the utility of the presented methods. Firefox and ImageMagick were able to open all generated files and display them as expected. However, GIMP sometimes had problems with images that deviated from the standard.