Objective

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Data hiding is a well-known method used to protect secret information or provide authentication. Thus

we can embed secret data into carriers

such as images and videos. After the embedding

the carriers can still perform as they were. Data hiding has many advantages over traditional encryption methods. Encryption algorithm can be regarded as a function used to transform secret data into cipher texts

and secret data and keys are parameters. However

making cipher texts understandable remains a challenge. If cipher texts are captured by an attacker

the attacker can easily notice that the texts must be processed

although he plain texts cannot be generated immediately. This situation may cause security issues. Such issues cannot happen in data hiding because the data-hiding process does not influence carrier readability. Humans also hardly notice the difference between cover and stego carriers. As a way of reversible data hiding

pixel permutation has a good ability of anti-gray histogram steganalysis but with low payload capacity. An adaptive reversible data-hiding algorithm based on pixel permutation is accordingly proposed.

Method

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First

a more efficient embedding unit structure than the conventional 2×2 pixel block structure is proposed. A cover image can be densely segmented

which provides a condition for improving payload capacity. For every embedded bit

the proposed triangular embedded pixel pair consists of 3 pixels. The specific shape and position of all embeddable pixel pairs (EPPs) are determined by the keys shared between sender and receiver. Second

EPPs are screened from all pixel pairs to avoid embedding secret bits in the pixel pairs that may cause a significant decline in image quality. Adaptive precoding is conducted according to the characteristic of gray trend of EPPs

such that the 0

1 distribution in the binary sequence that represents all EPPs is as asymmetrical as possible. This adaptive precoding process effectively improves Huffman coding compression ratio to enhance payload capacity. Huffman coding is a method of compressing a sequence using the frequencies of different arrays. This method spends a minimal storage on arrays with a high frequency and adopts long code words to represent arrays that seldom occur. Finally

data are embedded into the cover image by permuting the two end pixels of the corresponding EPP bit by bit. Data extraction is the inverse process of data embedding. Receivers can obtain all correct EPPs according to the keys and two pixel pair embedding conditions

and secret data can be easily extracted by using a location map. Eight different gray-scale images are randomly selected from a standard gray-scale test image data set for the experiments. A key is created by a pseudo random number generator. For every cover image

we test the payload capacity and PSNR of a full-load stego image. The histograms of cover and stego images are shown to prove the histogram robustness of the proposed scheme.

Results

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Experimental results demonstrate that the PSNR of the proposed scheme is improved by approximately 32% and its payload capacity is increased by more than 95% compared with those of a non-adaptive data-hiding algorithm with a 2×2 pixel block structure. The proposed scheme also keeps the robustness of gray-scale histograms before and after the data-hiding process. Notably

adaptability plays a decisive role in capacity improvement.

Conclusion

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This study presents a reversible information-hiding algorithm with high embedding capacity and a capability of anti-gray-scale histogram steganalysis. We construct an efficient embedding unit and differentiate embeddable regions for the characteristics of different cover images. The confidentiality of all embedding units is guaranteed by a key. The experimental results show that the proposed algorithm greatly improves payload capacity by adaptive precoding process and a special triangular pixel structure. Moreover

imperceptibility is kept in a high level by using appropriate EPP-filtering criteria. The proposed scheme considers the visual and statistical invisibilities of the data-hiding process. Consequently

the scheme provides a high security level for secret information when providing large embedding capacity of cover images. For practical applications

the proposed scheme can be utilized in secret information transmission

storage

and privacy protection. Although the payload capacity of the proposed scheme has been greatly enhanced compared with that of the same type of reversible data hiding methods

it remains lower than that of the methods without providing histogram robustness. Therefore

improving capacity while keeping confidentiality in a high level is worth of further research.