978-1-5386-0475-5/17/$31.00 ©2017 IEEE Comparison of Secret Color Image Sharing Based on XOR Operation in RGB and YCbCr Color Model Rinaldi Munir School of Electrical Engineering and Informatics Institut Teknologi Bandung (ITB) Bandung, Indonesia rinaldi.munir@itb.ac.id Abstract—Visual cryptography schemes for color images usually is performed by expanding every pixel in the secret image into some subpixels in the shared images. Wang’s schemes are secret image schemes based on XOR operation without expansion of the pixels so that size of shared images are equal to size of the secret images. The schemes are applied to RGB images. In this paper, a method to perform the schemes with minimal computation is proposed. The secret images in RGB color is transformed to YCbCr color model, and computation is performed only on Y component. The comparison of computation between RGB and YCbCr has been done. The results show that by applying the Wang’s schemes in YCbCr can minimize the computation without loss of quality. Keywords—visual cryptography, color images, Wang’s schemes, RGB, YCbCr I. INTRODUCTION Visual cryptography is a kind of cryptography for visual information (image). It encodes (encrypts) a secret image into some transparancies (also called as shares). It decodes (decrypt) information directly by human visual system without using a computer at all. The secret information in the image is decoded by stacking the transparancies and then we see the information visually. Visual cryptography is claimed to be perfectly secure because a share contains no information about the secret image. This concept of visual cryptography firstly was published 1994 by Naor and Shamir [1]. In the simple case, a secret image is split into two shares, and to reconstruct the secret image, the two shares are stacked together (Fig. 1). The scheme is then extended to k out of n sharing scheme or we call it as (k, n) threshold scheme. In the threshold scheme, a secret image is split into n shares, every participant get an individual share. To decode the secret information, k or more participants stack their shares. However, no information obtained when k – 1 participants stack their shares. Fig. 1 Example of Naor and Shamir’s visual cryptography scheme Originally, Naor and Shamir applied their scheme on black and white image only. In their scheme, a pixel is split into two subpixels. A pixel is encrypted as two subpixels in each of the two shares. In 1996 and 1997, Rijmen & Preneel [2] and Verheul & Tilborg [3] developed the schemes that applied to color secret images. Next, some researchers developed more advanced visual cryptography schemes based on steganography by sharing and hiding the color image into multiple meaningfull images [4, 5]. All of secret image sharing schemes, in encoding phase, expand one pixel into multiple sub-pixels. For example, one pixel is expanded into four subpixels. As the consequence, size of the shared image increases four times larger than the original image. In addition, the reconstructed image loss of contrast. It contains noises that make it look like a noisy image. In 2005, Wang et all proposed a secret color image sharing with easy encryption, decryption, lower computation complexity, without pixel expansion and better contrast [6]. The secret image can be reconstructed by using XOR operation. In computer XOR is a elementer and simple operation that make it apply to cryptographic system. There are two threshold scheme that was be proposed (later we call them “the Wang’s schemes”). The first scheme is (n, n) threshold scheme with no pixel expansion and the relative contrast difference is 1, which the secret image is reconstructed based on XOR operation. The second scheme is (2, n) with no pixel expansion and the relative contrast difference is ½., the secret image can be reconstructed by using XOR and AND operation. The (n, n) threshold scheme can recover the image same exactly with the original image, whereas in the (2, n) scheme the recovered image contains noises.