Objective

2

Traditional search-based coverless information hiding usually searches natural and unmodified carriers that contain appropriate secret vectors to transmit secret information. However

for natural images and texts

the ability to express irrelevant secret information is low. Thus

the hidden capacity is low. Search-based coverless information hiding cannot avoid the dense transmission of massive carriers that attract attacks easily and results in distorted secret information. Texture-based coverless information hiding usually divides sample texture image into several blocks

creating the mapping relationship between texture blocks and secret segments. Although such methods generated stego texture images by sample texture synthesis

they can only generate simple unnatural texture images. Mosaic-based information hiding can generate meaningful stego images

but in essence

such methods are modification-based information hiding

and they inevitably leave modification traces in carriers and have no authentication strategy. Therefore

verifying the authenticity of recovered secret information is impossible. Moreover

mosaic-based information hiding typically uses the LSB(least significant bit)-based modification embedding strategy for a large embedding capacity to embed the transformation parameters. The LSB-based embedding strategy is sensitive to attacks

and the embedded information is easily lost when suffering attacks. To address these problems

this study proposed a generative camouflage method combined with block rotation and photo mosaic.

Method

2

In the embedding

the proposed method transformed grayscale images into circle images with pseudo-random angles by user keys to construct photo mosaics and then generated random integer coordinate sequence to determine the hidden positions of secret bit strings. For each hidden position

a circle image related to user key and position was placed to express secret bits

and then the random angle related to user key and position was added for authentication. For each non-hidden position

a circle image similar to cover image pixel was set to conceal the secret image

and the added random perturbation angle precluded distinguishing hidden and non-hidden positions. The deviation caused by the placement of the circle image was scattered around unprocessed pixels by the error diffusion method to form the stego photo mosaic image. In the extraction

coordinates for all secret bits in the stego photo mosaic image were obtained by user key

and circle images expressing secret bits were fetched. The centroid of each circle image that expresses secret bits was normalized into the right half-axis and then identified by the strategy of centroid rotation matching. Each circle image index was found to extract the hidden secret bits by the minimum quadratic difference distance. To authenticate the correctness of each circle image fetched secret bits

the factual and theory angles were compared

where the minimum quadratic difference distance of the factual angle

and the theory angle was computed by the user key

and the fetched secret bits. If the factual angle equals the theory angle

then the extracted secret bits are correctly recovered.

Result

2

The proposed method regards each circle image as a hidden unit and only involves the rotation of circle images without any modification. The added random rotation angles also provide the security assurance for the specific user to extract secret information and conduct angle authentication. Only the user with the correct user key can eliminate the influence of random rotation angles and extract the correct secret information with its corresponding authentication information. Moreover

the proposed method uses photo mosaic to hide secret information. The method is robust in anti-attacks and has good authentication accuracy. At the same time

the proposed strategy relies entirely on the user's key and has high security. The experiments show that secret information can be completely recovered only by the correct key. Whether one or more user key parameters are changed

the changed user key parameters lead to the inaccurate extraction of secret information. In addition

in case of JPEG compression attacks with a quality factor of 50~80 and random rotation angle attacks

the embedded secret information can be completely recovered. Even with salt-and-pepper noise attacks of 8%~20%

the error rate(ER) of extracted information remains approximately 5%. The authentication success rates of the restored information are all above 80%.

Conclusion

2

The proposed method uses randomly placed circle images to express secret information without any modification. The method can use photo mosaic to generate meaningful hidden carrier images and avoid the high cost of database creation and searching of the search-based coverless information hiding. The proposed method combines the parameter mapping and error diffusion mosaic image generation strategy to place stego circle images in their corresponding positions and then generate the stego mosaic image

avoiding the dense transmission of massive carriers in search-based coverless information hiding. Compared with the texture-based coverless information hiding

the proposed method can generate meaningful stego photo mosaic images

avoiding the generation of meaningless texture images in texture-based coverless information hiding. Even during attacks

each circle image expressed secret bits and its placement angle are not easily lost. The proposed method can extract secret information by centroid rotation matching and random rotation angle strategy based on user keys and improve the accuracy of secret information recognition.