## 1 Introduction  
Networks are ubiquitous in modern life. More and more things are increasingly digital, such as photos, videos, music, documents, personal information, and so on. Therefore, how to protect digital information is a hot issue. Cryptography and steganography are two popular technologies used to protect digital products. For cryptography, a key is used to encrypt data into meaningless numbers, then we can use the same key or another to decrypt. Common encryption methods are advanced encryption standard (AES), data encryption standard (DES), RSA and MD5. In general, cryptographic technologies provide a certain level of security, but cannot maintain security when the ciphertext is decrypted. Therefore, steganography technologies have been developed. Steganography technologies can be classified into watermarking and data hiding [1]. Digital watermarking technology, in general, can be divided into two categories [2], visible and invisible watermarking. A visible watermark's advantage is the human eye can discern it. No algorithm is needed to view the information that represents data sources or the owner. The disadvantage of a visible watermark is that the image is changed by the watermark. It is easily overwritten or removed by signal processing technology. Watermarking techniques can be divided into two types: fragile and robust watermarks. A fragile watermark is primarily used to protect the integrity of the image. The slightest modification to the media with a fragile watermark results in the destruction of the watermark. A missing watermark denotes tampering. Robust watermarking can survive a designated class of transformations. An example of a robust watermark application is a watermark to carry copy and access control information. The media may be compressed, cropped or otherwise transformed, but the watermarked information survives. Utilizing digital signal processing and digital imaging technologies to hide secret data without reducing the quality of the cover image is called data hiding. This technique is not readily apparent and hides information in any form (text, images, video). Data hiding has two techniques: spatial domain and transform domain. This kind of data hiding technology has very high image quality and is undetectable by the human eye. The technology of data hiding can be classified into two types: one is irreversible data hiding, and the other is reversible. The difference is reversible data hiding is lossless and can reconstruct the original cover image from the stego image after the secret message is extracted. The watermarking technology always modifies pixels in the frequency domain [3], but pixels modified in the frequency domain contribute to more distortion. Therefore, data hiding technology usually occurs in the spatial domain for less distortion than watermarking technology and quick processing. For data hiding in the spatial domain, least significant bit (LSB) replacement is a classic scheme [4], but the capacity of embedded data is one bit per pixel (bpp), and it has no security. The exploiting modification direction (EMD) [5] scheme can embed \(1.16\) bpp for two pixels in each group. Data compression [6] can save storage space and speed up network transmission. In other words, the raw data are processed through various mathematical algorithms to reduce data storage space. This reduced amount of data is transmitted. Then, the decompression operation can recreate the original data at the receiver. There are two types of data compression. One is lossless compression, such as PCX, GIF, TIFF, TGA and PNG image formats, ZIP, RAR data compression technology, run-length encoding, Huffman coding and Lempel-Ziv-Welch (LZW). The other is lossy compression, such as JPEG (Joint Photographic Coding Expert Group), VQ (Vector Quantization) [7; 8; 9] and SMVQ [10; 11] (Side Match Vector Quantization). Therefore, to achieve the smaller secret message size, the data compression technology is a good solution.