an improved rko algorithm for colour visual

An Improved RKO Algorithm for Colour Visual Cryptography and Authentication

Sona A J PG Scholar: Dept. of CSE

Vidya Academy of Science and Technology Thrissur, India

[email protected]

Nitha K P Asst. Prof, Dept. of CSE

Vidya Academy of Science and Technology Thrissur, India

[email protected]

Abstract—Authentication is essential to verify a person’s identity and it is critical in providing access control. Researches are developing day by day in order to make the process of authentication more secure and to eliminate loopholes. The power factor of an authentication system greatly depends on its encrypted secret keys. Visual cryptography is a powerful cryptographic technique used for secret sharing. The latest and simplest method in visual cryptography to encrypt color images is RKO algorithm. The weakness of this algorithm is that, the Key Share constructed using the RKO is vulnerable to cryptanalytic attacks since it is generated using simple Exclusive-OR operation. The paper discusses about the different visual cryptographic techniques used for secret key generation and how it can be used to improve the security of authentication systems. A slightly modified version of RKO algorithm is also suggested.

Keywords— Security, Authentication, Visual cryptography, Colour Visual Cryptography, RKO Algorithm

I. INTRODUCTION Visual cryptography is a powerful cryptographic technique introduced by Noar and Shamir [7] in 1994, used to encrypt secret

visual information such as printed text, handwritten materials and images. Here the decryption is performed by the human visual system, without using computers and thus visual cryptography scheme eliminates complex computations required in decryption process.

The secret binary image is a collection of black and white pixels, and each pixel is treated individually and it is encrypted into n number of cipher images called as shares. Each share contains m sub-pixels which are either black or white.

Fig. 1. Basic 2-0ut-of-2 Visual Cryptography

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Fig. 2. This structure can be represented as an n x m Boolean matrix S = [sij], where sij=1 if the jth sub-pixel in the ith share is black, and sij=0 if the jth sub-pixel in the ith share is white. When the shares, i1, i2 … are superimposed one over other, such that it properly aligns the sub-pixels, the original secret is reconstructed by Boolean "or" operation. [7].

Fig. 3. The grey level of this reconstructed secret is proportional to the Hamming weight H (V) of the "OR"- ed m-vector V. This is interpreted by the visual system of the users as black, if H (V) >= d and as white, if H (V) < d – αm for some fixed threshold 1 <= d < =m and relative difference, α>0[7].

II. LITERATURE SURVEY This section discusses about various visual cryptographic techniques and their applications in improving the security of

authentication.

A. Basic 2-out-of-2 Visual cryptography In 2-out-of-2 visual cryptography, the secret is encrypted into two shares, share1 and share 2, and only when these two shares

are superimposed, the secret is revealed. The shares are meaningless and appear as a random collection of black and white pixels, giving no clue about the original secret image. In earlier work of visual cryptography, each pixel in the secret image is represented using 4 pixels in each share to maintain the aspect ratio of the original image. As a result, the size of the generated shares is 4 times the original image, that is, the secret was encrypted with the expansion ratio of 1:4 (and in later works, 1:2). This expansion ratio indicates that if original secret image is of size AXB then the shares generated will have size 4AX4B that is, expansion ratio of 1:4. If the expansion ratio is 1:2, the shares generated will have a size of 2AX2B [4]. This drawback is called as Pixel-expansion.

B. K-out-of-N Visual Cryptography Adi Shamir [8] generalized the problem as follows: Let D be the secret data and the aim is to divide D into ‘n’ components D1,

D2 …Dn so as to satisfy the following conditions: Anyone knowing of any k or more Di components can easily compute D. D cannot be computed by knowing any k-1 or less Di pieces only. Such a scheme is called a (k, n) threshold scheme. Given a secret visual message, it generate ‘n’ encrypted shares and the

secret can be reconstructed only if any k (or more) of shares are stacked or superimposed one over the other.

C. N-out-of-N Visual Cryptography Here, the original secret image is encrypted into ‘n’ [8] number of shares, but to reconstruct the secret image, all the ‘n’ shares

are required.

III. VISUAL CRYPTOGRAPHIC TECHNIQUES AND AUTHENTICATION In authentication and security applications, Visual cryptography is used to generate encrypted keys and to give an extra layer of

protection. This section describes about advanced visual cryptographic techniques which are used in authentication applications. For example, [9] describes about an authentication application which takes fingerprint inputs using visual cryptography

methods and gives a model of authentication with ID card. In the Capturing stage, the fingerprint samples are collected using sensing device and characteristics are extracted from this raw

sample which is then stored in the database for comparison during verification. Each of these can considered as a secret image. A random, unique dummy share is created and saved securely in the database. The shares of the participants are created from this dummy share and their fingerprint images using visual cryptography and it is hidden inside the photograph on the ID card. At the time of verification, the system will reconstruct the fingerprint image enrolled during registration using the dummy share and participant share. This image is compared with the fingerprint provided for verification.

The method attempted to solve two major problems regarding the fingerprint authentication systems, that is, falsification and the costly maintenance of large fingerprint database.

As an extra layer of protection to the existing authentication system, visual cryptography techniques can be also used to ensure security to the iris templates [10] stored in the database as in the case of fingerprint authentication system.

The disadvantage of using the grey-scale input image is that, it does not offer perfect reconstruction of original image at the time of verification and the reconstructed secret image will be darker when compared to the original secret image. Sometimes it may degrade the performance of the authentication system. By inserting additional white pixel patterns [11], the contrast of the generated secret image can be improved to get a good quality image. Another method is to directly enhance [5] the contrast of the reconstructed image so as to completely eliminate the greying effect. These methods have no pixel expansion and have better contrast compared with traditional Visual Cryptography Schemes.

Apart from the basic schemes, researchers have developed many improvements in the area of visual cryptography which can be used to ensure more protection in authentication mechanisms. It includes the color visual cryptography, hierarchical visual cryptography, and extended visual cryptography.



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A. Color Visual Cryptography The earlier work of colour visual cryptography was using colour decomposition and half-toning. The process of Half-toning

transforms an image with greater amplitude resolution to one with lesser amplitude resolution. Or in other words, half-toning changes a continuous tone image into dots, varying in either size, shape or spacing, where continuous tone imagery contains an infinite range of colours or greys [12].

By half-toning the input image, the size of the database can be reduced. For colour images, there are two alternatives for applying half toning which produces same results at the end: (i) Divide the colour image into component images: cyan, magenta and yellow and treat each component image is as a grey scale image. Half toning and visual cryptography are applied independently to these components. (ii) Apply direct colour half toning, and then perform the separation into component images. Visual cryptography is then applied to each component image independently.

According to the theory of colour decomposition, every colour on a colour image can be decomposed into three primary colours: Cyan, Magenta, and Yellow [6]. And using the halftone technique, a grey-level image can be transformed into a binary image which then can be used to input to visual cryptography.

This concept is used to build an authentication system for bank credit cards in [13] which take customer image and handwritten signature as input to 2-out-of-2 visual cryptography for colour images. The advantage of this scheme is that even if the customer’s card is lost, it cannot be misused and it provides two-level protection.

In a novel hybrid approach of Colour Visual Cryptography and image encryption, which known as RKO technique [1], the input image is encrypted into two secret shares: a random dummy share and a key share. The method involves there steps: Random Share generation, Key Share generation and Overlapping of two shares. In Random Share generation, a random (dummy) share is generated by taking any random value for Red, Green and Blue channels for each pixel. In Key share generation, the key share is produced by performing pixel-by-pixel XOR operation of random share with original image.

The size of these shares maintains a 1:1 ratio with the original image. Also, two key shares of the same image are never identical since two random shares are never same. In Overlapping the superimposing of the shares is done by performing XOR operation on random share and key share, and the original image is reconstructed. This algorithm provides a perfect reconstruction property so that there is no loss in the quality of the image. [3] proposes a novel authentication system based on RKO technique. The system makes use of customer image and scanned handwritten signature image to generate secret shares and thus provides 2-level security.

B. Hierarchical Visual Cryptography

Hierarchical visual cryptography (HVC) provides the extension of encryption into multiple levels. [3] Initially the secret is encrypted into exactly two shares (share1 and share2). Each share is then taken independently and encrypted and produced four shares: share11, share12 (from share1), share21, share22 (from share2). From these four shares, randomly three shares are selected to generate the key share [5].

The pixel-expansion affects hierarchical encryption of secret because the size of the shares will come in multiples of 4. The algorithm [4] describes a technique to produce shares without pixel-expansion, that is, the original secret image size is can be kept unchanged at all levels of hierarchical visual cryptography.

Here, before encryption, the original secret image is resized to a particular size, which will be multiple of 4. After that, each pixel block of size 2X2 is selected for encoding independently, starting from top left corner of secret input image using standard equations defined by the algorithm.

Pixel-expansion and additional processing needed in hierarchical visual cryptography can be avoided by the effective use of RKO technique [2].

C. Extended Visual Cryptography

Random black n white shares are suspicious, and catch the attention of attackers. Meaningful shares are used in order to avoid such situations. A (k, n)-Extended Visual Cryptography scheme takes a secret image and ‘n’ original images as input and produces ‘n’ encrypted shares with some similarity to the original images and they satisfy the following t conditions:(i) Any k out of n shares can recover the secret image. (ii) Any less than k shares cannot reconstruct the secret image. (iii) All the shares are meaningful images; encrypted shares and the reconstructed secret image are color images [16] [17] [18].

[14] proposes an improved color visual cryptography based encryption method using with error diffusion half-toning techniques. The use of error diffusion and half-toning will diffuse away the noise in the present pixels and therefore the visual appearance of the encrypted and decrypted images will be improved.

The presence of Visual Information Pixels (VIP) [16], which are the pixels on the encrypted shares that have same color values that of the original images. They give the encrypted shares meaningful appearance. In each of the m sub-pixels of the encrypted share, there will be λ numbers of VIPs, denoted as ci and the remaining (m-λ) pixels gives the message information of



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the secret image. VIP’s are substituted at the same bit position in sub-pixels in the three color channels. Therefore they represent exactly the same colors of the original image.

Thus, each sub-pixel carries secret information along with visual information avoiding extra pixels needed for producing meaningful shares, which causing pixel expansion. After encrypting sub-pixels of three color channels corresponding to each message pixel, random permutation operation is performed and a set of encrypted sub pixels for three color channels also permuted at the same time to preserve the VIP synchronization.

IV. FINDINGS

The findings after studying various cryptographic techniques are shown in the below table. It focuses on the greying effect and pixel expansion observed in the reconstructed image and the number of encryption levels.

TABLE I. VISUAL CRYPTOGRAPHIC TECHNIQUES

No.

Technique Studied

Greying effect

Pixel expansion

Levels of Encryption

1 Basic Visual Cryptography[7]

Yes Yes Single

2 (k,n) Threshold Scheme [8]

Yes Yes Single

3 Visual cryptography with dummy shares of fingerprint [9]

Yes Not Specified Single

4

Iris recognition system [10]

Yes

Not

Specified

Single

5

Contrast enhancement, additional pixel patterns [11]

No

Yes

Single

6

HVC, Expansion-less shares [4]

Yes

No

Double

7

HVC, Contrast enhancement, Expansion-less shares [5]

No

No

Double

8

RKO Technique [1]

No

No

Single

9

Hierarchical RKO[2]

No

No

Double

V. IMPROVED RKO ALGORITHM RKO technique is the existing simplest method to generate secret shares from a colour image. The greatest weakness of

this algorithm is that, it is possible to rebuild the Random share as well as the Original image from the Key share through cryptanalysis.

For example, let ‘Orig’ be any pixel value from an 8-bit RGB image input, ‘Rand’ be the corresponding Random share pixel value and ‘Key’ be the generated value of corresponding Key share through XOR operation

Orig = 1010 1010 Rand = 0001 0010



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Key = 1011 1000

TABLE II. EX-OR GATE : TRUTH TABLE

Input-1 Input-2 Output

0 0 0 0 1 1 1 0 1 1 1 0

Consider the Least significant bit of Key share pixel, which is 1. The bitwise XOR operation gives a 1 whenever the 2

inputs are same (i.e., either (0, 0) or (1, 1)). Therefore, it can be predicted that, the corresponding pixel values of both the random share and original image is either 0 or 1. Similarly, bitwise XOR operation gives a 0 if the input pair is either (0, 1) or (1, 0) and can be predict the corresponding original image and random pixel values.

This weakness can be avoided using Random permutation technique as follows: Separate the Red, Green, and Blue Channels of input colour image. Substitute random values for Red, Green, and Blue values. Concatenate the three channels to form the Random share. Perform bitwise XOR operation on each pixel of Random share with corresponding pixel of original image to create Key

share. Apply Random permutation on Key share. To regenerate the original image, apply inverse permutation on Key share and perform bitwise XOR operation of Key

share and Random share. Random permutation makes the cryptanalysis impossible and thus improves the security of the algorithm. To rebuild the secret

inverse permutation is performed on the Key share.

VI. EXPERIMENTAL ANALYSIS The improved RKO algorithm is tested with different images of varying size and dimensions. The quality of the output image is compared with the original image using three parameters such as MSE (Mean Square Error), PSNR (Peak Signal-to-Noise ratio) and NC (Normalized correlation) []. The results where MSE=0, PSNR= and NC=1 for all inputs. That is, reconstructed images were the exact copies of the input images, thus it is functioning as the original RKO algorithm. The following figures show the outputs of improved RKO algorithm, when it is tested using the benchmark image ‘peppers.png’ in MATLAB.

Fig. 4. Original Image

Fig. 5. Random Share



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Fig. 6. Key Share

Fig. 7. Permuted Key Share

Fig. 8. Reconstructed Key Share

Fig. 9. Reconstructed Input Image

VII. ADVANTAGES The visual cryptographic technique is an efficient scheme to share the secret as it is really simple to implement. It doesn’t

require a decryption scheme to reconstruct the secret. So that no need of cryptographic knowledge to decrypt the hidden information. When we want to share a secret, we can send it through FAX or E-mail, and only less computational cost is required since the secret message is recognized by human eyes and not cryptographically computed.

VIII. DISADVANTAGES When we use the visual cryptography to create biometric cards, it is vulnerable to stolen smart card attacks that is, anyone with the card can access the system. To prevent such attacks, we must make use of multi-factor authentication. For example, the card along with a secret PIN number can improve the security.

IX. APPLICATIONS Visual cryptography is a powerful methodology that can be used effectively to generate secret keys in biometric

authentication systems such as finger print, iris, and handwritten signatures. It has been widely using to ensure protection against



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the phishing websites. It is also applicable in the area of digital watermarking. Another important application is in the field of steganography, where secret is hidden inside a cover object which may be an image, a video or text file.

X. CONCLUSION The visual cryptographic techniques are effective to be used to generate secret keys in authentication techniques. The major

drawbacks were due to the pixel expansion and greying effect. It can be avoided by using colour visual cryptography and improved RKO technique. Extra protection can be achieved if RKO based colour visual cryptography is combined with steganography.

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