# Reversible Data Hiding in Encrypted Images Based on the Mixed Multi-Bit Layer Embedding Strategy
## 1 Introduction
Compared with the previous VRAE schemes, RRBE schemes are more convenient for the high embedding capacity, reversibility, and security. Under the RRBE framework, the sender pre-processes the cover image before encryption to reserve room or generate a new image format conducive to hiding. In previous studies, classifying image blocks and pixels by using some rules is a common technical means. [PERSON] et al. [23] classified image blocks by smoothing function calculation to reserve the LSBs of pixels in blocks. [PERSON] et al. [24] divided image blocks according to the pre-designed threshold and reserved room by compressing some blocks' LSB. [PERSON] et al. [25] classified the cover image into blocks with different scales. Then, the lowest two bits of each pixel were used to vacate room. The pixels were sorted according to the predefined fixed classification mode in [26; 27]. For the above methods, the adaptive classification method [23; 24; 25] has the advantage of mining the hidden space as much as possible but requires uncertain auxiliary information. The fixed classification methods [26; 27] have fixed costs but limit the amount of hidden space. Prediction technology is also applied through high performance [28; 29; 30]. Most significant bit (MSB) prediction was proposed in [31; 32; 33; 34] based on the high correlation of the adjacent pixel values of the cover image, which significantly improves the embedding capacity and the image recovery quality. Interpolation techniques [26; 35; 36], compression [24; 37; 38], and HS [39] have also been further studied and applied to encrypted images. These schemes all use a single hider and maintain the amount of data that is always kept consistent with the size of the cover image during transmission. In addition, in recent years, some scholars have proposed multiple hiders schemes [40; 41; 42] based on secret sharing, which has also promoted the development of this field. The above technologies have improved recovery quality and embedding rates. However, there may be problems with reserving room and how data embedding can be achieved after encryption. It can be found that some problems exist in previous studies, such as the fact that the secret data cannot be extracted completely and accurately in some algorithms [14; 15; 16], so complete reversibility cannot be achieved. The embedding capacity needs to be improved [19; 20; 23; 33], but the image recovery quality will be affected. The joint schemes at the receiver [14; 26; 27; 37] and others need to process the data in order for the operation to be simple but low in flexibility; the separable schemes such as [17; 18; 19; 20; 21; 22; 23; 24; 25; 33; 36] are crucial to achieving independence in data extraction and image recovery but also increase the complexity. Therefore, RDH-EI is more challenging than traditional RDH. Balancing the embedding rate, the accuracy of secret data extraction, the quality of image recovery, and separability is a technical challenge. In this paper, all the challenges mentioned above are considered. A separable RDH-EI scheme based on a mixed multi-layer embedding strategy is proposed to improve the embedding capacity under error-free data extraction conditions. The scheme belongs to the RRBE framework. The contributions of the proposed scheme are as follows:  
* A two-round block segmentation method with different sizes is proposed, which extends the number of hidden blocks and is conducive to providing embedding room;
* Adaptive multi-bit embedding (AMLE) and fixed bit layer embedding (FMLE) strategies are designed to embed all data jointly, which can compress auxiliary data to reserve more room for data embedding;
* Separability and full reversibility can be guaranteed;
* Decrypted images maintain satisfactory quality at maximum embedding capacity. The rest of the paper is organized as follows. Section 2 shows the related works. Section 3 describes the specific scheme. Section 4 gives the experimental results and performance analysis. Section 5 presents the conclusion.