GRAPICS DESIGN OF A CRYPTOGRAPHIC PROTOCOL USING RECURSIVE PAIRS & BITWISE OPERATION ON A BLOCK (RPBOB) P.K.Jha 1 , J.K.Mandal 2 , G.P.Sharma 3 1,3 School of Engineering and Technology Purbanchal University, Biratnagar, Nepal E-mail: amp_jha@yahoo.com , gopalpsh@yahoo.com 2 Dean, Faculty of Engineering & Management, Kalyani University,Kalyani,West Bengal Email: jkmandal@rediffmail.com Abstract:The technique considers a message as binary string on which a Design of a cryptographic protocol using Recursive pairs & Bitwise operation on a block (RPBOB) is performed. A block of n bits is taken as an input stream, where n varies from 4 to 256, from a continuous stream of bits and the technique operates on it to generate the intermediate encrypted stream. This technique directly involves all the bits of blocks in a boolean operation and a session key. The same operation is performed repeatedly for different block sizes as per the specification of a session key to generate the final encrypted stream. It is a kind of block cipher and symmetric in nature hence, decoding is done following the same procedure. A comparison of the proposed technique with existing and industrially accepted RSA and TDES has also been done in terms of frequency distribution and non homogeneity of source and encrypted files. Key words: Design of a cryptographic protocol using Recursive pairs & Bitwise operation on a block (RPBOB), Cipher text, Block cipher, Session Key. I. Introduction In the emerging area of the cryptography [1, 2, 3, 4] strong protocols are used effectively in the strategy of protecting confidential information during its transmission over a network. Information is encrypted at the senders end using an encryption protocol and a key. On reaching at the destination point, the task of decryption is executed using a decryption protocol along with a key to regenerate the source information. Encryption and decryption are in nut shell termed as cryptography. On the basis of the keys used in the entire process, there exists two category of cryptography. In private key cryptography, a single key is used during encryption and decryption and is to be kept private. In public key cryptography, different keys are used during encryption and a private key during decryption. Section 2 of the paper deals with the principle of this paper. A proposal for key generation and vulnerability is described in section 3. Results are given in section 4. Analysis about the technique is made in section 5. Conclusions are drawn in section 6 and references are drawn in section 7. II. The Technique This technique operates in two phases: a. First phase encrypt the message using Cascaded Arithmetic Operation on Pairs of bits of a streams. A stream of bits is considered as the plaintext. the plaintext is divided into a finite number of blocks, each having a finite fixed number of bits like 8/ 16/ 32/ 54/ 128/ 255. This technique is then applied for each of the blocks in the following way. The rules to be followed for generating cycles are as follows: 1.Consider any source stream of a finite number, where N=2 n , n =3 to 8. 2. Make the source stream into paired form so that a pair can be used for the operation. 3. Make the modulo-2 addition (X-OR) between the first and second pair, second and third pair, third and fourth pair and so on of the source stream, to get the first intermediate block. Any intermediate block in the recursive process may term as intermediate encrypted block for that source block and any block can be taken as the input for the second phase. b. Second phase encrypt the output of the first phase by using Bitwise Operation on Blocks This technique, considers the encrypted message from the first phase(here third encrypted block is taken) as a stream of finite number of bits N, and is divided into a