Proposing A Symmetric Key Bit-Level Block Cipher
  A novel bit level block cipher based symmetric key cryptographic technique using G.C.D is proposed in this research paper. Entire plain text file is read one character at a time and according to the binary representation of ASCII value of the characters, entire plain text file is divided into n number of 16 bit blocks. Then an agreed-upon symmetric key file is formed by dividing each 16 bit block into two 8 bit sub blocks and by using Greatest Common Divisor (G.C.D) operation among them. The key size is 40 times the number of 16 bit blocks present in the plain text file as each block produces a key of size 40 bits. The character corresponding to the G.C.D value for each block is stored in the cipher text file which is sent by the sender to the receiver along with the symmetric key file. From the symmetric key file and the cipher text file, the receiver by applying reverse procedure, gets back the original plain text file. This technique has several merits, some of them are formation of symmetric key file dynamically, achievement of 50% compression rate in the cipher text file, better security in terms of brute force attack and applicability of the technique for a large number of files of different size and type. 
Sarbajit Manna, Saurabh Dutta
cs.CR