a public key watermarking scheme for image authentication - IJIEEE

International Journal of Industrial Electronics and Electrical Engineering, ISSN: 2347-6982

Volume-2, Issue-5, May-2014

A PUBLIC KEY WATERMARKING SCHEME FOR IMAGE AUTHENTICATION 1

JAYASHREE S PILLAI, 2SREERAMAMURTHY T. G

1

Research Scholar, MTWU, 2Professor, Acharya Institute of Management & Sciences Email: [email protected], [email protected]

Abstract- Conventional digital signature schemes for image authentication encode the signature in a file separate from the original image, thus require extra bandwidth to transmit it. Watermarking techniques embed some information in the host image. In this paper, a public key based digital signature scheme for image authentication is proposed. The scheme extracts feature value from the pixels of the of the original image. This feature is used to compute the signature which is embedded into the DCT of the image as watermark, avoiding additional signature file. It is inspired by Wong’s Public Key Watermarking scheme which was later enhanced by Baretto. The watermarked image demonstrated high perceptual quality and robustness. It also proved to be robust against many common image processing operations and JPEG compression. The scheme not only can verify the authenticity and the integrity of images, but also can locate the illegal modifications. Keywords- block based, digital signature, feature extraction, hash functio, jpeg, DCT

I. INTRODUCTION Watermarking techniques consider the image as a communication channel. The embedded watermark, usually imperceptible, may contain either a specific producer ID or some content-related codes that are used for authentication. Both the techniques are useful to identify the source of the image and alterations that occur after the insertion of the signature or watermark.

Section III explains the proposed algorithm, Section IV discusses the experimental results and Section V summarizes the conclusion. II.

RELATED WORK

Wong proposed a public key fragile marking technique for image integrity verification. The host image and the watermark are both images. A digest is generated from the host image using a hash function which is encrypted using a public key system to create the watermark. The watermark is embedded in the LSB of the image such that when the correct detection side information and unaltered marked image are provided to the detector, a bi-level image chosen by the owner is observed. The main goal of this scheme is to ensure the image’s ownership and integrity. When a watermarked image is received, we can use the extraction procedure to judge if the watermarked image has undergone altering. Wong’s scheme is subject to the counterfeiting problem. An attacker can produce a fake yet correctly watermarked image by launching the cut-and-paste attack, the birthday attack or other counterfeiting attacks. Wong’s scheme can be easily circumvented by the cut-and-paste attack and the Birthday attack

The need for image authentication and integrity is well emphasized in several imaging applications including legal use of images, archiving and sharing of medical images, copyright in news broadcasting systems and commercial transactions. In all these applications, there is an inherent requirement to verify the integrity and authenticity of the media. Robust, fragile and semi fragile techniques are can be used for watermarking based on the type of application, but for image authentication, fragile and semi fragile techniques are normally used. Fragile watermarking techniques may not be able to tolerate even the slightest modifications to an image while robust watermarking techniques may be intolerant to intentional modifications. A lot of research has already been done in the area of authentication of images using watermarking. Image authentication includes two aspects, one is enforcing copyright ownership and the other is ensuring image integrity. The watermarking procedure can be carried out in the spatial or transform domain. Embedding in the transform domain as DCT, DFT or DWT makes the image robust to common and incidental image processing operations like compression and filtering whereas images watermarked in the spatial domain are very fragile to them. Any watermarking system has to satisfy certain general requirements like robustness, sensitivity, capacity, localization and imperceptibility

III.

The rest of the paper is organized as follows. Section II gives a brief of the previous work undertaken,

In this paper, a semi fragile watermarking technique is proposed which is intolerant to incidental image

To solve the problem of tampering, Baretto et al. adopted the strategy of hash block chaining – HBC1 and HBC 2 to achieve inter block dependency among the image blocks. The watermark for each block is not dependent only on that block but also on the subsequent block. THE PROPOSED SCHEME

A Public Key Watermarking Scheme for Image Authentication 46



processing operations like jpeg compression and filtering but at the same time robust to intentional attacks like cut and paste and modifications.

2. Perform steps 1 – 6 of the watermark generation procedure to obtain the computed watermark W ~ = {w ~ , w ~ , … … … . . w ~ }

The watermarking procedure is explained in 3 steps 1) Watermark generation 2) Watermark embedding procedrue 3) Watermark extraction procedue and 4) Watermark verification procedure

IV.

EXPERIMENTAL RESULTS

The proposed algorithm was implemented in Mat lab 7 on grey scale images of various sizes. Padding was done to images that were of uneven sizes. Images of different formats like tiff, bmp and png were used in the experiments.

Watermark generation procedure 1. Segment the host image X into n blocks. X represents each block and the mean M of each block X is calculated 2. The feature value F for each block is F = M ⁄M , 0≤ r ≤ n−1 (1)

The test images were segmented into blocks of size 8 and 16. From the experiments, it was concluded that block size of 16 ensures efficiency in time often resulted in a better PSNR value. The quality metrics used to measure the quality of the watermarked image is PSNR – Peak Signal to Noise Ratio, PCC – Pearson Correlation Coefficient, Image quality index and SSIM – structural similarity. The metrics are calculated for the original and watermark image.

The calculation of F for each block follows a zig zag manner and for the last sub block, i.e., r = n − 1, F = M ⁄M (2) 3. SHA1 is used to calculate the digest of the feature value F which is then encrypted using the owners private key to generate the digital signature S which is going to be the content based watermark 4. Repeat steps 2 – 3 for computing the watermark for each of the blocks. This set will form the signed watermark W = {w , w , … … . w }

TABLE 1 MEASURE OF THE QUALITY METRICS OF THE WATERMARKED IMAGES

Watermark embedding procedure The watermark is embedded into the mid frequency coefficient of the DCT of the image. 1. Calculate the DCT D of each block X 2. Select the mid frequency coefficient at the selected location(p,q) in each block and replace it with the modified coefficient computed using the watemark watermark DCT(p, q) = sign DCT(p, q) ∗ (α ∗ w) Where α is the calculated value for embedding strength and the sign of the coefficient is retained 3. Perform inverse DCT 4. Repeat steps 2 for each of the blocks

In this algorithm, the average PSNR value is >53 which indicates high quality watermarked image. The average values of PCC, image quality index and SSIM is very close to 1 which indicates that the watermark is imperceptible.

The resulting image X ∗ is the watermarked image Watermark extraction procedure 1. Perform block based DCT on the received watermarked image X ∗ 2. Extract the embedded watermark from the mid frequency coefficient of each block |DCT(p, q)| w = α and store it to get the extracted watermark

TABLE 2 RESULTS OF WATERMARK EXTRACTION AFTER INCIDENTAL SIGNAL PROCESSING OPERATIONS OF CAMERAMAN IMAGE

W ∗ = {w ∗ , w ∗ , … … . w ∗ } Watermark verification procedure 1. After removing the embedded watermark from the DCT of the received image, perform inverse DCT to get the image without the watermark A Public Key Watermarking Scheme for Image Authentication 47



extraction of the watermark is also quite high. The algorithm is able to identify tampered portions of the image with accuracy.

JPEG 90% JPEG 80% JPEG 70% Fig: 1- Output of JPEG compression of varying quality factors

REFERENCES

It was observed that the watermarked image is quite tolerant to normal image processing operations like JPEG compression at various quality levels, Gaussian and salt and pepper noise and median filtering for grey scale images. The robustness of the watermarking technique is quite high where 8 bits per block is embedded. In a 256x256 image, using block size 16, 4096 bits are embedded and in 512x512 images, 16384 bits are embedded s watermark. Detection of tampering of watermarked image The original images were subject to intentional attacks like cut and paste attacks and modification attacks using Adobe Photoshop, Paint and other image processing tools. The watermark extraction algorithm could identify the regions of attack

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Fig: 2- Tampering attacks on livingroom (512x512) and Lena (256x256)

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TABLE 3 DETECTION OF TAMPERING ATTACK ON THE WATERMARKED IMAGES

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CONCLUSION This paper presents a semi fragile watermarking technique using image content as the watermark. No additional watermark is needed. The ratio of mean values of each block is used as the feature value and are embedded in the mid frequency coefficients of each image block. The test results indicate the watermarking algorithm is robust to incidental image processing operations like JPEG compressions and other as indicated in Table 2 and is still able to authenticate it. The quality of the image after

[13] N. Memon, P. W. Wong, and S. Member, “A Buyer – Seller Watermarking Protocol,” IEEE Trans. IMAGE Process. vol. 10, no. 4, pp. 643–649, 2001. [14] G. K. Ci, E. Janu, K. Cius, and H. Schumann, “Tamper-Proof Image Watermarking , Based on Existing Public Key Infrastructure,” vol. 16, no. 1, pp. 1–18, 2005. [15] S. Shivani, “Image Authentication and Restoration using Block- Wise Fragile Watermarking based on k-Medoids Clustering Approach,” 2nd Int. Conf. Work. Emerg. Trends Technol., pp. 44–51, 2011.

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