International Journal of Computer Applications (0975 – 8887) Volume 96– No.12, June 2014 18 Performance Analysis of Turbo Coding with AES for CCSDS Standard Shajina.V M.Tech student Department of Electronics Engineering Pondicherry University P.Samundiswary Assistant Professor Department of Electronics Engineering Pondicherry University ABSTRACT Turbo codes enable reliable communication over power- constrained communication channels at close to Shannon’s limit. Further, the performance of the communication system is enhanced by incorporating security mechanism. Security is described as the process of minimizing the vulnerabilities of assets or resources. National Institute of Standards and Technology (NIST) chose Rijndael algorithm as Advanced Encryption Standard (AES) for security mechanism due to the enhanced security, performance efficiency, ease of implementation and flexibility. To ensure security and error correction together in the system, the encrypted data is given to the turbo encoder and at the receiver the data is decoded and decrypted back. Hence the system works well without much reduction in the bit error rate with the added advantage of security. In this paper, turbo coding with and without AES are simulated and Bit Error Rate (BER) analysis is done by varying SNR and number of iterations. The simulation is done using C. General Terms Channel coding, Turbo coding, Security, Algorithms Key words AES, Turbo coding, Max-log-MAP algorithm 1. INTRODUCTION The purpose of a telemetry system is to convey measured information from a remotely located data generating source to users located in space or on Earth reliably and transparently. The task of channel coding is to encode the information sent over a communication channel in such a way that errors can be detected and/or corrected by the decoder while receiving the information in the presence of channel noise [1]. In addition, cryptography is the science of information and communication security, enabling the confidentiality of communication through an unsecured channel. It protects against unauthorized parties by preventing unauthorized alteration of use [2]. Turbo code is a very powerful error correcting technique, which enables reliable communication with BER close to Shannon limit [3]. It uses more bits of redundant data in order to achieve better reliability. The theoretical Maximum A posteriori Probability (MAP) algorithm is one of the decoding methods that is computationally complex and inaccurate estimation of the noise variance. The logarithmic version of the MAP algorithm [4] and the Soft Output Viterbi Algorithm (SOVA) are the practical decoding algorithms for implementation in real time systems. These algorithms are less sensitive to SNR mismatch and inaccurate estimation of the noise variance. SOVA has the least computational complexity and has the worst BER performance among these algorithms, while the Log- MAP algorithm [4] has the best BER performance equivalent to the MAP algorithm. However, it has the highest computational complexity. Encryption uses a cryptographic system to transform a plaintext into a cipher text, using keys. The Advanced Encryption Standard (AES) is the winner of the contest, held in 1997 by the US Government, after the Data Encryption Standard (DES) was found too weak because of its small key size and the technological advancements in processor power [5]. Fifteen algorithms were accepted in 1998 and based on public comments, the pool was reduced to five finalists in 1999. In October 2000, a slightly modified version of the Rijndael algorithm [6] is selected as the forthcoming standard out of these five algorithms. The algorithm is named as Rijndael based on the names of its two Belgian inventors, Joan Daemen and Vincent Rijmen that is a Block cipher, which means that it works on fixed-length group of bits, which are called blocks. It takes an input block of a certain size, usually 128, and produces a corresponding output block of the same size. The transformation requires a second input, which is the secret key. It is important to know that the secret key can be of any size (depending on the cipher used) and that AES uses three different key sizes: 128, 192 and 256 bits [6]. In this paper, Max-Log-MAP algorithm is used for turbo decoding which is less complex than the Log-MAP algorithm but it performs very close to the Log-MAP algorithm. Further, AES is incorporated along with the turbo codes such that good error correction and security can be assured. Also BER performance is analysed. This paper is organized as follows; the block diagram is explained in section2.The turbo coding with optimized Max-Log-MAP decoding is explained in section 3. In Section 4, Rijndael algorithm for AES is explained. The analysis of the results is given in section 5. Finally, the work is concluded in section 6. 2. BLOCK DIAGRAM OF PROPOSED SYSTEM Nowadays, there is a strong need of designing communication systems with excellent BER performance and high levels of privacy, especially in wireless communications. The transmission of encrypted information over a noisy channel presents an error propagation effect, which degrades the BER performance of the system. Here, combined error control coding and encryption schemes based on iteratively decoded error-control turbo codes and AES algorithm is developed. These make a given communication system with excellent BER performance and encryption capabilities. Figure 1 shows the basic block diagram of the work done in this paper.