IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 24, NO. 4, APRIL 2014 695 A New Secure Image Transmission Technique via Secret-Fragment-Visible Mosaic Images by Nearly Reversible Color Transformations Ya-Lin Lee, Student Member, IEEE, and Wen-Hsiang Tsai, Senior Member, IEEE Abstract —A new secure image transmission technique is pro- posed, which transforms automatically a given large-volume secret image into a so-called secret-fragment-visible mosaic image of the same size. The mosaic image, which looks similar to an arbitrarily selected target image and may be used as a camouflage of the secret image, is yielded by dividing the secret image into fragments and transforming their color characteristics to be those of the corresponding blocks of the target image. Skillful tech- niques are designed to conduct the color transformation process so that the secret image may be recovered nearly losslessly. A scheme of handling the overflows/underflows in the converted pixels’ color values by recording the color differences in the untransformed color space is also proposed. The information required for recovering the secret image is embedded into the created mosaic image by a lossless data hiding scheme using a key. Good experimental results show the feasibility of the proposed method. Index Terms—Color transformation, data hiding, image encryption, mosaic image, secure image transmission. I. Introduction C URRENTLY, images from various sources are frequently utilized and transmitted through the internet for various applications, such as online personal photograph albums, con- fidential enterprise archives, document storage systems, medi- cal imaging systems, and military image databases. These im- ages usually contain private or confidential information so that they should be protected from leakages during transmissions. Recently, many methods have been proposed for securing image transmission, for which two common approaches are image encryption and data hiding. Image encryption is a technique that makes use of the natural property of an image, such as high redundancy and strong spatial correlation, to get an encrypted image based on Shannon’s confusion and diffusion properties [1]–[7]. The encrypted image is a noise image so that no one can obtain Manuscript received May 9, 2013; revised July 21, 2013; accepted Septem- ber 16, 2013. Date of publication September 25, 2013; date of current version April 2, 2014. This research was supported in part by the NSC, Taiwan, under Grants 101-3113-P-009-006 and 102-2218-E-009-003, and in part by the Ministry of Education, Taiwan, under the five-year Project of “Aiming for the Top University” from 2011 through 2015. This paper was recommended by Associate Editor W. Zhu. The authors are with the National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: yllee.cs98g@g2.nctu.edu.tw; whtsai@cis.nctu.edu.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCSVT.2013.2283431 the secret image from it unless he/she has the correct key. However, the encrypted image is a meaningless file, which cannot provide additional information before decryption and may arouse an attacker’s attention during transmission due to its randomness in form. An alternative to avoid this problem is data hiding [8]–[18] that hides a secret message into a cover image so that no one can realize the existence of the secret data, in which the data type of the secret message investigated in this paper is an image. Existing data hiding methods mainly utilize the techniques of LSB substitution [8], histogram shifting [9], difference expansion [10]–[11], prediction-error expansion [12]–[13], recursive histogram modification [14], and discrete cosine/wavelet transformations [15]–[18]. However, in order to reduce the distortion of the resulting image, an upper bound for the distortion value is usually set on the payload of the cover image. A discussion on this rate- distortion issue can be found in [19]. Thus, a main issue of the methods for hiding data in images is the difficulty to embed a large amount of message data into a single image. Specifically, if one wants to hide a secret image into a cover image with the same size, the secret image must be highly compressed in advance. For example, for a data hiding method with an embedding rate of 0.5 bits per pixel, a secret image with 8 bits per pixel must be compressed at a rate of at least 93.75% beforehand in order to be hidden into a cover image. But, for many applications, such as keeping or transmitting medical pictures, military images, legal documents, etc., that are valuable with no allowance of serious distortions, such data compression operations are usually impractical. Moreover, most image compression methods, such as JPEG compres- sion, are not suitable for line drawings and textual graphics, in which sharp contrasts between adjacent pixels are often destructed to become noticeable artifacts [20]. In this paper, a new technique for secure image transmission is proposed, which transforms a secret image into a meaningful mosaic image with the same size and looking like a preselected target image. The transformation process is controlled by a secret key, and only with the key can a person recover the secret image nearly losslessly from the mosaic image. The proposed method is inspired by Lai and Tsai [21], in which a new type of computer art image, called secret-fragment-visible mosaic image, was proposed. The mosaic image is the result of rearrangement of the fragments of a secret image in disguise of another image called the target image preselected from a 1051-8215 c  2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.