CRYPTOGRAPHIC METHOD FOR DIGITAL DOCUMENT
V.ARUNA1 Assistant Professor(ECE),
Park college of Engineering and technology, coimbatore, [email protected]
9791737714
Abstract- Due to the recent development of computers and computer networks, huge amount of digital data can easily be transmitted or stored. However, transmitted data in networks or stored data in computers may easily substitute by enemies Therefore; security is one of the most important technologies in computerized society. Hence, an authentication method based on secret sharing scheme via PNG image and bin mapping is proposed to authenticate the digital document images. In these proposed method, an authentication data is created for each block of document image which is transformed into several secret shares using Shamir’s secret sharing scheme and then embedded into the alpha channel plane. Then the alpha channel plane is combined with the original cover image to form a PNG image. In verifying the integrity of the image the authentication signals are computed from the current block which is then compared with the extracted shares from the alpha channel plane for the respective block. If the authentication data are not match then that block is marked as tampered block and repaired by reverse secret sharing scheme.
Keywords: authentication, secret sharing, png image, alpha channel, document image.
I. INTRODUCTION
Biometric Data are very confidential information of user. But providing high security to these document images is a challenging serious task. By recent advanced digital technologies it is possible to modify the contents of these digital document images. So, it is important to make method to solve image authentication problem, particularly for document images. In this paper, a new method for authentication of document images with self repairing capability for fixing tampered image data is proposed. The binary like grayscale image is taken as input image. In general, the image authentication problem is difficult for a document image because of its simple binary nature which leads to perceptible changes after authentication signals are embedded in the image pixels. Such changes will cause possible attack from attackers. A good solution to this image authentication thus should take into account not only the security issue of preventing image tampering, but also keeping the visual quality of the resulting image. In this method, an authentication method is proposed which deals with binary-like grayscale document images instead of pure binary ones, and solves simultaneously the problems of image detection and quality keeping.
II. PROPOSED METHOD
In (k, n)-threshold secret sharing scheme, a secret message is changed into n shares and given to n participants is shown in figure 1.
2.1 Algorithm for (k, n)-threshold secret sharing:
(1) Choose randomly a prime number p that is larger than d.
(2) Select k-1 integer values c1, c2, c3…ck -1 within the range of 0 through p-1.
(3) Select n distinct real values x1, x2, x3,……,xn.
(4) Use the following k-1 degree polynomial to compute n function values F (xi), called partial shares for i=1,2,3,...,n. i.e.,
2.2Algorithm for Secret Recovery:
Step1: From the k shares (x1, F(x1)), (x2, F(x2)),…… (xk, F (xk)), the generated polynomial equation is,
Step 2: Using Lagrange’s interpolation to obtain
Step 3: Compute c1 through ck-1 by
2.3 Alpha Channel of PNG Image
PNG means Portable Network Graphics. PNG provides a replacement for GIF. When a color source color is blended with background color image, alpha value determine the resulting color. If the alpha channel is transparency; the source color is invisible, if the alpha channel is opaque, the source color overwrites the destination color, if the value is in between, which creates a translucent effect.. The alpha channel with 100%
opacity layer is added to the document image for its transparency.
III. GENERATION OF STEGO IMAGE The block diagram for the generation of stego image is shown in Fig 2.
Fig 2. Generation of stego image
Step 1: Apply thresholding method to input image I to binarize I Step 2: Transform I into a PNG image with an alpha channel Iα
Step 3: Divide the input document image I into 2×3 block Bb of Ib.
Step 4: Generate a 2-bit authentication signal for each block, s = a1a2 with a1 = p1 p2p3 and a2 = p4p5p6.
Step 5: Concatenate the eight bits of a1, a2, and p1 through p6, divide into two 4-bit; and transform into decimal m1 and m2.
Step 6: Perform the Algorithm (2, 6) threshold secret sharing scheme qi = F(xi) = (d + c1xi)mod p (5)
Step 8: The first two pixels is replaced by secret shares, remaining four pixels are embedded with key K.
IV. AUTHENTICATION AND REPAIRING The block diagram of the authentication of stego image is shown in Fig 3.
Step 1: Separate the alpha channel s and input cover image from received image.
Step 2: Apply thresholding to binarize the input image.
Step 3: Divide the binary input image in 2*3 blocks.
Step 4: Processed the alpha channel as block wise, from six pixels of each block, subtract 238 and apply (2,6) reverse secret sharing scheme for first two pixels,
(6)
Fig 3.Authentication of Stego image
Step 5: Transform the secret and co-efficient values into 4 bit binary , concatenate into two 4 bit binary segments.
Step 6: Take the first six pixels of each block and compared and verify its authenticity.
Step 7: If the block is not matched, it is considered as tampered block and repaired.
Step 8: In repairing process, the remaining four pixels of each block are collected.
Step 9: For each block, take two pixels, and apply (2, 6) reverse secret sharing scheme,
Step 10: Transform the values into binary segments, concatenate 4 bit binary digits. Hence, the attacked document image is repaired.
V. SIMULATED RESULTS
The proposed method is applied to various digital document images. The simulation is verified by the tool Matlab. The document image of size 150*340 is taken as the input image is shown in fig 4.
Fig 4. Input Image
+
=
Fig 5. Adding Alpha Channel
By image editing software, alpha channel is added to the input image is shown in the fig 5. The alpha channels have 100% opacity. The Statistics of experimental results of attacks is shown in the table 1 with various parameters.
Fig 6. Attacked Image
By image editing like super imposing, and the attacker able to change the content of the image is shown in the fig 6. The attacked image is fully repaired by reverse secret sharing scheme is shown in the fig 7.
Fig 7.Recovered image
Alpha channel is an important feature of PNG image.. Full transparency is represented as 0% or 0, full opacity is represented as 1.0, 100% or 255.
The performance of the proposed method is explained in terms of the five parameters ratio.
(1) Tampering ratio = ( number of tampered blocks) / (total number of blocks);
(2) Detection ratio = (number of detected blocks) / (number of tampered blocks);
(3) Repair ratio = ( number of repaired blocks)/ ( number of detected blocks);
(4) False acceptance ratio = ( number of tampered blocks marked as untampered)/( total number of tampered blocks);
(5) False rejection ratio = ( number of untampered blocks marked as tampered)/( total number of untampered blocks).
By these parameters, it is verified that the proposed method is good for recovering the tampered blocks and without any distortion to the input document image.
VI. MERITS OF PROPOSED METHOD
The proposed method Provide a pixel-level repairing of tampered image.
By secret sharing scheme, the proposed method can survive various attacks.
Conventional image authentication methods usually embed authentication data into the original image.
VII. COMPARISON WITH OTHER METHODS
A comparison of the capabilities of the proposed method with other existing methods is shown in Table 2.
Table 2: comparison of proposed method with other methods
The proposed method has the capability of repairing the tampered parts of an image. The methods with tampering localization like Yang and Kot [5], Tzeng and Tsai [8], and the proposed method, the proposed method provides a better authentication. Specifically, the method in [5] needs macro-blocks to yield pixel flippabilities for embedding authentication data. In the case of using micro blocks, Tzeng and Tsai’s method [8]
has a higher chance of possibility to generate noise pixels as mentioned in [6], and so they conducted experimental results with the larger block size of 64×64. The proposed method is the only one which makes use of the alpha channel instead of the bit plane.
VIII. CONCLUSION
The authentication method based on secret sharing scheme is proposed. Both the authentication signal and the content of block have transferred into partial shares by Shamir’s secret method which are generated into alpha channel plane. Stego image is considered in the form of PNG format and from embedding the partial. A part of stego image authentication has been tampered if authentication signals does not match for self repairing of tamped block the reverse scheme is used. Enhancing the security of the data embedded in the alpha channel plane were also proposed. Experimental results have been shown to prove the effectiveness.
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