Date Published: February 27, 2019
Publisher: Public Library of Science
Author(s): Wien Hong, Xiaoyu Zhou, Der-Chyuan Lou, He Debiao.
In this paper, we propose an efficient method for authenticating the absolute moment block truncation coding (AMBTC) compressed images with the capability to recover tampered blocks. The existing methods may not be able to detect some types of intentional tampering. Meanwhile, the tampered blocks are only recovered by their means, causing an unpleasant mosaic-like appearance. The proposed method classifies image blocks into groups according to their similarities, and the group information is recorded for the recovery purpose. The multiple copies of the group information are scrambled and embedded into the bitmap of smooth blocks. The dominant portions of quantization levels are adjusted to generate a set of authentication code candidates. The codes with the minimal distortion are embedded into the least significant bits (LSBs) of the quantization levels. The tampered blocks can be recovered by averaging those untampered ones with the same index. The experimental results show that the proposed method not only achieves an excellent marked image quality and detectability, but also offers a satisfactory recovered result.
The rapid development of computing and software technologies has made people easy to enhance their photo qualities, create collages, and perform many other photo editing applications. In contrast, these modern techniques can also be used to tamper with digital images maliciously. Therefore, the development of image authentication schemes for protecting the integrity of digital images has been increasingly becoming important. In general, image authentication schemes can be classified into the signature-based  and fragile watermark-based [2–3] approaches. The signature-based authentication approaches obtain the signatures from images and store the obtained signatures in a trusted third party. To authenticate the image, the signatures stored in the trusted third party are extracted to verify the integrity of the image. The fragile watermark-based approaches embed fragile watermarks into the image to construct a marked one. Since the embedded watermarks are fragile, tampering of a marked image will damage the embedded watermarks. Therefore, the presence of the tampering can be detected.
We briefly introduce the AMBTC compression technique in this section. Hu et al.’s method, which will be extensively compared with the proposed method, is also presented in this section.
Hu et al.’s method uses the quantization levels and bitmaps of smooth blocks to embed authentication codes and recovery codes, respectively. Their method successfully detects and approximately recovers the tampered blocks, and the detection mainly relies on Eq (2). However, for any integer ρ, substituting a˜i±ρ×2w or b˜i±ρ×2w into Eq (2) also obtains the same p˜i. Therefore, their method fails to detect the tampering by adding or subtracting ρ×2w to the marked quantization levels. Moreover, the tampering of marked bitmaps also cannot be detected by their method because the modification of bitmaps does not alter the extracted authentication codes. Besides, the embedment of the w-bit authentication codes aciw into the quantization levels may cause a significant distortion for a large w. Therefore, the overflow and underflow problems can possibly occur. However, Hu et al.’s method provides no mechanisms to deal with these problems. Moreover, all pixels of a tampered block are recovered by a single mean value. As a result, the unpleasant mosaic-like effect is apparent, particularly in image edges.
In this section, we conduct several experiments to evaluate the performance of the proposed method and compare the results with some related works. A total of eight grayscale images of size 512×512 are used as the test images (Fig 4). In these images, the first six images are obtained from the USC-SIPI image database , while the last two images are taken from a modern digital camera. These test images are compressed using the AMBTC compression technique with a 4×4 block size. The compressed codes are then used to embed the authentication and recovery codes.
In this paper, we propose an efficient authentication method with the capability of recovery for the AMBTC compressed image. The blocks of the AMBTC image are classified into groups according to their similarities. Several copies of the group indices are embedded into the bitmaps of smooth blocks. The dominant portions of the quantization levels, the bitmap, and other required information are hashed to generate the authentication codes. The generated authentication codes are then embedded into the LSBs of the quantization levels. The proposed method successfully detects a variety of tampering, while the tampered blocks are recovered using similar patterns. Experimental results reveal that the proposed method not only outperforms Hu et al.’s and other related methods, but also achieves a very satisfactory marked image quality.