Image Encryption Using AES with Modified … image security, encryption is used. Various algorithms for encryption have been proposed such as AES, RSA, IDEA [1, 2]. These algorithms

International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064

Impact Factor (2012): 3.358

Volume 3 Issue 7, July 2014 www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

Image Encryption Using AES with Modified Transformation

Harleen Kaur1, Reena Mehla2

1Student, Department of Electronics & Communication, Kurukshetra University, India 2Assitant Professor, Department of Electronics & Communication, Kurukshetra University, India

Abstract: With the increased growth of internet, extensive exchange of data over the network has been made. Safe storage of digital images and secure transmission over the network is always been required, so as to protect the confidential data from unauthorized access. High transmission rate of the images using limited bandwidth makes the standard algorithms unsuitable for encryption. The work proposed in this paper is focussed on a new algorithm, which requires less encryption time, less computational power requirement and maintain a sufficient level of security. This paper proposes a modified version of AES. Various modifications have been made in the transformation steps of AES. Experimental results for encryption time using proposed AES have been shown and compared with original AES. Keywords: Advanced Encryption Standard (AES), Image Encryption, Computational time analysis, Modified mixing column transformation, Modified shift row transformation 1. Introduction With the development of internet technology, applications such as multimedia data, video conferencing, broadcasting are being used more and more. For image security, encryption is used. Various algorithms for encryption have been proposed such as AES, RSA, IDEA [1, 2]. These algorithms are mostly used in text data. For multimedia data these algorithms are not suitable for real-time applications. This paper proposes a new modified version of AES algorithm. The modification is made on the various transformation steps in AES algorithm. Basically the modification is focussed on Shift row transformation, mixing column transformation and key expansion modification. 2. AES Algorithm AES algorithm uses three cryptographic keys of 128, 192, 256 bits and accordingly AES is referred to as AES-128, AES-192, AES-256, operates in 10 rounds. At the beginning of encryption, the input bytes are mapped to state array. In the end, the final value is mapped to output array bytes. AES has the following transformation steps: 1. Sub Byte Transformation 2. Shift Rows Transformation 3. Mix Column Transformation 4. Add Round Key Transformation A. Sub Byte Transformation In this step each byte of state is substituted using S-box, which is constructed by multiplicative inverse and affine transformations. Here S-box is used as a look up table. B. Shift Rows Transformation In this step the bytes are shifted cyclically to the left. The first row is kept un-shifted. Second, third and fourth rows are shifted cyclically 1, 2, 3 bytes to the left respectively.

C. Mix Column Transformation This transformation step operates on state column-by-column. Each column is treated as a four term polynomial. Every column is considered as GF (28) polynomial and multiplied by polynomial a(x) modulo x4+1, where a(x) = {03}x3+{01}x2+{01}x+{02} D. Add Round Key Transformation In this step simple XOR operation is done between each byte of state and each byte of sub key. Key expansion transformation: AES algorithm takes the encryption key and performs a key expansion transformation to generate sub keys. Key expansion generates 11 sub key arrays of 16 words of 8 bits denoted by wi (except w0), previous wi-1, Rcon and S-box are used. Rcon[i]=[RC[i],0,0,0] with RC[1]=2*RC[i-1]

Table 1: Rcon Values i RC[i] 1 01 2 02 3 04 4 08 5 10 6 20 7 40 8 80 9 1b 10 36 11 6c 12 d8 13 a6 14 4d

Above table shows the different round constant values for RC[i], i=1 to 14

Paper ID: 020141015 360

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Paper ID: 020141015 362

International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064

Impact Factor (2012): 3.358

Volume 3 Issue 7, July 2014 www.ijsr.net

Licensed Under Creative Commons Attribution CC BY

Author Profile Harleen Kaur, Student (M.Tech), Doon Valley Institute of Engineering and Technology, Kurukshetra University, Haryana, India. Reena Mehla, M.Tech (ECE), Assistant Professor, Doon Valley Institute of Engineering and Technology, Kurkshetra University, Haryana, India

Paper ID: 020141015 363