IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 __________________________________________________________________________________________ Volume: 03 Issue: 03 | Mar-2014, Available @ http://www.ijret.org 653 GMDES: A GRAPH BASED MODIFIED DATA ENCRYPTION STANDARD ALGORITHM WITH ENHANCED SECURITY Debajit Sensarma 1 , Samar Sen Sarma 2 1 Department of Computer Science & Engineering, University of Calcutta, Kolkata, West Bengal, India 2 Department of Computer Science & Engineering, University of Calcutta, Kolkata, West Bengal, India Abstract Cryptography is one of the prime techniques of secured symbolic data transmission over any communication channel. Security is the most challenging and essential aspects in today’s internet and network applications. Thus, design of a secure encryption algorithm is very necessary which can protect the unauthorized attacks. An encryption algorithm is computationally secure if it cannot be intruded with the standard resources. The algorithm proposed here is graph based. Its efficiency surpasses the standard DES algorithm in general. Graphs can be used for designing block ciphers, stream ciphers or public-key ciphers. The algorithm is graph automorphism based partial symmetric key algorithm and it is not fully depended on secret key and produces different cipher text by applying same key on the same plain text. Keywords: DES, Graph Automorphism, Hamiltonian Cycle, Encryption, Decryption -----------------------------------------------------------------------***----------------------------------------------------------------------- 1. INTRODUCTION Cryptography is a science which studies the techniques for secure communication in the presence of intruders or unauthenticated access. It is about constructing and analyzing protocols that overcome the influence of intruders. Cryptography converts the original message in a non-readable format and sends the message over an insecure channel. The original message is called plain text. Disguising the plain text to hide its original contents is called encryption. The non- readable format of the plain text after encryption is called cipher text. The process of reverting the cipher text to its corresponding plain text is called decryption process. For both encryption and decryption process key is used. It is used with plain text at the time of encryption and with the cipher text at the time of decryption. Cryptography provides number of security goals to ensure the privacy of the data. The goals of the cryptography are- Confidentiality, Integrity, Availability, Authenticity, Non Repudiation, Access control [2]. In cryptography the encryption algorithms can be classified into two broad categories- Symmetric key and asymmetric key encryption. In Symmetric key cryptography the key used for encryption and decryption is same. Thus, the key must be distributed through the secure channel before transmission started. These types of algorithms are highly depended on the nature of the key. DES, Triple DES, AES, RC4, RC6, BLOWFISH etc are the example of symmetric key algorithms. In asymmetric key cryptography two different keys are used for encryption and decryption, they are private and public key. The public key is available to all in the network. The sender who wants to transmit message, encrypts the message with receiver‟s public key and only the authorized receiver can decrypt the message with its private key. RSA is the example of asymmetric key cryptography. According to [15] symmetric key algorithm is faster than asymmetric key algorithm and also memory requirement of the former is lesser than the later. In this paper a graph based modified DES algorithm is proposed. It is depended on the Hamiltonian cycle and the automorphism [3, 4] property of the 4-cube graph. Here, an arbitrary Hamiltonian cycle of a 4-cube graph is used as a secret key and sixteen different Hamiltonian cycles of the non Automorphic graphs of the given 4-cube graph is used as the sub keys for sixteen rounds like classical DES. It is a partial symmetric key algorithm based on the block cipher. The main advantage of the proposed algorithm is, like other symmetric key algorithms it is not fully depended on the secret key, rather it also depended on the sub keys, which are remain encrypted with the private key of the sender and stored in a secure mapping table. When a receiver wants to decrypt the message, it will send request to the sender. The sender uses Zero Knowledge Protocol [6] to verify the receiver. If the receiver is authorized, then sender will decrypt the sub keys of the secret mapping table with the sender‟s private key. Then only that particular receiver can decrypts the cipher text. Another advantage is that, this algorithm produces different cipher texts for a single key and single pain text, which also decreases the probability of various malicious attacks. The paper is organized as follows- In section 2 some preliminaries are given, Section 3 explain a brief review of related literature, Section 4 illustrates the proposed algorithm, in section 5 a short example is given, section 6 gives some