IOSR Journal of Computer Engineering (IOSR-JCE) e-ISSN: 2278-0661,p-ISSN: 2278-8727, Volume 19, Issue 3, Ver. III (May - June 2017), PP 75-79 www.iosrjournals.org DOI: 10.9790/0661-1903037579 www.iosrjournals.org 75 | Page RSA-A Symmetric Key Cryptosystem Shreedatta Sawant 1 , Vaishnavi Kamat 2 , Vishal Snedan Robertson 3 , Sneha Kamat 3 , Anish Thali 3 , Anuj Shetgaonkar 3 1 (Asst. Prof., Computer Engineering Department, AITD, Goa University, India) 2 (Head, Asst. Prof. Computer Department, AITD, Goa University, India) 3 (Students, Computer Engineering Department, AITD, Goa University, India) Abstract: Data security has become a growing concern in this digital age. Current cryptographic algorithms either compromise on computational speed or data security. We put forth a variable key length 1024-bit symmetric cryptosystem which encrypts and decrypts text data in the form of 256-bit blocks at a time. It is fast as well secure and it is completely dependent on the key. Keywords: Cryptosystem; Encryption; Decryption; Ciphertext; Cellular Automata; Block Cellular Automata; Margolus neighborhood; Rule generator, Add key I. Introduction Over the past few decades’ wireless communication has become the most widely accepted form of communication. Wireless technology has made communication possible across the globe within seconds. But as to every advantage there is a disadvantage with the faster speed of communication comes the disadvantage of threat to security of the data which is transmitted over the wireless lines. Data which is transmitted over the wireless channel can be read and modified by a third party listening to the channel. This is a severe disadvantage as it may lead to loss and misuse of critical information sent over the communication line. Encryption is one of the efficient ways used to secure data over the channel, in which messages are converted into human unreadable format such that only authorised people can get access to the original message. This message or information to be transmitted is known as plaintext and the message in human unreadable format is known as cipher text. For securing plaintext, various cryptographic systems are available which are divided into two categories Symmetric cryptographic systems and Asymmetric cryptographic systems. In a symmetric cryptographic system, a single key is used by the sender and the receiver to encrypt the plaintext and to decrypt the cipher text. Symmetric algorithm includes AES (Advanced Encryption Standard), Twofish, etc. In Asymmetric cryptographic system two separate keys are used for encryption and decryption of plaintext one of them is a shared public key and the other is a private key such as RSA. II. Literature Survey A cellular automaton is a collection of cells on a grid which changes its state with time depending on a set of rules & on the states of neighbouring cells. A block cellular automaton is a special kind of cellular automaton in which the grid of cells is split into non-overlapping blocks and the rules are applied to a whole block at a time. The neighbourhood of a cell is the nearby cells. One of the neighbourhoods used in block cellular automata is the Morgolus neighbourhood, in which the grid is divided into 2-cell blocks which are shifted by one cell along both axes on alternate time steps. Block Cellular Automata Rules are defined as a 1D array containing a permutation of the numbers 0 to 15. In 2014, Said Bouchkaren and Saiida Lazaar [1] explain an algorithm to use Block Cellular Automata in cryptography. The algorithm uses a single rule. Each iteration of transformation using the rule has 2 phases. Phase 1: Create sub-blocks of size 2x2 starting from cell at row 0 & column 0. Decimal equivalent of each block is obtained and is used as an index in the Rule to obtain the value to be mapped back to the block. Phase 2: Create sub-blocks of size 2x2 starting from cell at row 1 & column 1. Decimal equivalent of each block is obtained and is used as an index in the Rule to obtain the value to be mapped back to the block Forward Block Cellular Automata (FBCA) performs in each iteration Phase 1 followed by Phase 2. Reverse Block Cellular Automata (RBCA) performs in each iteration Phase 2 followed by Phase 1. In 2004, Marcin Seredynski and Pascal Bouvry under the section Reversible Cellular Automata have stated that the number of reversible rules that can be used in cryptography should be a very large number and that they should behave in a complex manner. Elementary 1D CA has only 256 rules in total of which only 6