Abstract—With the fast evolution of digital data exchange, security information becomes much important in data storage and transmission. Due to the increasing use of images in industrial process, it is essential to protect the confidential image data from unauthorized access. In this paper, we analyze the Advanced Encryption Standard (AES), and we add a key stream generator (A5/1, W7) to AES to ensure improving the encryption performance; mainly for images characterised by reduced entropy. The implementation of both techniques has been realized for experimental purposes. Detailed results in terms of security analysis and implementation are given. Comparative study with traditional encryption algorithms is shown the superiority of the modified algorithm. Keywords—Cryptography, Encryption, Advanced Encryption Standard (AES), ECB mode, statistical analysis, key stream generator. I. INTRODUCTION NCRYPTION is a common technique to uphold image security. Image and video encryption have applications in various fields including internet communication, multimedia systems, medical imaging, Tele-medicine and military communication. Many image-protection techniques are using vector quantization (VQ) as encryption technique (Chang et al., 2001; Chen and Chang, 2001). In Chang et al. (2001), VQ decomposes an image into vectors, which are then encoded and decoded vector-by-vector. Alternatively, Chen and Chang (2001) use VQ to divide desired images for encryption into a large number of shadows that are guaranteed undetectable to illegal users. Image and text cryptography has been achieved using chaotic algorithms (Fridrich, 1997; Sobhy and Shehata, 2001, Haojiang, Yisheng, Shuyun and Dequn Li 2005). A symmetric block encryption algorithm creates a chaotic map (Fridrich, 1997) for permuting and diffusing image data. For thorough encryption, the chaotic map is applied to the image, iteratively, multiple times. The chaotic algorithm of Sobhy and Shehata (2001) is based on the Lorenz system of equations. Both image and text data are encrypted Manuscript received March 4, 2007. This work was supported by Tunisian/French CMCU project and Sultan Qaboos University, Oman. M. Zeghid, M. Machhout, and R. Tourki are with the Electronics and Micro-Electronic Laboratory (LEME), Monastir, Tunisia. L. Khriji is with the Department of Electrical and Computer Engineering, Sultan Qaboos University, Muscat, Oman. He is on leave from the Institut Supérieur des Sciences Appliquées et de Technologie de Sousse (ISSATS), Tunisia (e-mail: lazhar@squ.edu.om). A. Baganne is with LESTER-University of South Brittany, Lorient, France. successfully, but knowledge of the system allows devising an optimization routine that discovers the key by output minimization. Phase encoding techniques exist for encrypting image data (Zhang and Karim, 1999; Park et al., 2001). Color image data is regarded in Zhang and Karim (1999), where a double-phase technique is utilized. Color images are encrypted from an indexed image and thereby decrypted back to its color format. The work of Wu and Kuo (2001) describes selective encryption based on a digital coefficients table. It was shown its limitation due to a less intelligible recovered image. Color and gray-scale images were considered in Koga and Yamamoto (1998), where a lattice-based extension to Visual Secret Sharing Scheme (VSSS) (Naor and Shamir, 1994) was developed. A hashing approach to image cryptography is taken in Venkatesan et al. (2000); wavelet representations of images are obtained, and a new randomized strategy for hashing is introduced. Several cryptosystems exist like as data encryption [3], steganography [14], digital signature (Aloka Sinha, Kehar Singh, 2003) and SCAN (S.S. Maniccama, N.G. Bourbakis 2004) have been proposed to increase the security of secret images. However, one common defect of these techniques is their policy of centralized storage, in that an entire protected image is usually maintained in a single information carrier. If a cracker detects an abnormality in the information carrier in which the protected image resides, he or she may intercept it, attempt to decipher the secret inside or simply ruin the entire information carrier (and once the information carrier is destroyed, the secret image is also lost forever). Another method is to encrypt image data, e.g., using DES (Data Encryption Standard). DES, however, is very complicated and involves large computations. A software DES implementation is not fast enough to process the vast amount of data generated by multimedia applications and a hardware DES implementation (a set-top box) adds extra costs both to broadcasters and to receivers. In order to tackle these problems systems based on advanced encryption standard (AES) where proposed. AES is very fast symmetric block algorithm especially by hardware implementation [7, 11, 12, 15]. The AES algorithm is used in some applications that require fast processing such as smart cards, cellular phones and image-video encryption. However, a central consideration for any cryptographic system is its susceptibility to possible attacks against the encryption algorithm such as statistical attack, differential attack, and various brute attacks. Block cipher symmetric algorithms; allow different ciphering mode [17]. Electronic CodeBook (ECB) is the most obvious mode; ciphered blocks is a function of the corresponding plaintext block, the algorithm and the A Modified AES Based Algorithm for Image Encryption M. Zeghid, M. Machhout, L. Khriji, A. Baganne, and R. Tourki E World Academy of Science, Engineering and Technology International Journal of Computer and Information Engineering Vol:1, No:3, 2007 745 International Scholarly and Scientific Research & Innovation 1(3) 2007 scholar.waset.org/1307-6892/7580 International Science Index, Computer and Information Engineering Vol:1, No:3, 2007 waset.org/Publication/7580