layfair cipher is the well-known multiple letter encryption cipher. Here the digraphs in the plaintext are treated as single units and converted into corresponding cipher text digraphs. However because of the drawbacks inherent in the 5 X 5 Playfair cipher which adversely affects the security we proposed a 6 X 6 Playfair cipher and then coupled it with Linear Feedback Shift Register based Unique Random Number Generator [1]. 6 X 6 Playfair cipher supports all 26 alphabets (A-Z) and 10 digits (0-9) which eliminate the limitation of 5 X 5 Playfair in which "i" and "j" both character could not appear at the same time [2, 3]. LFSR not only enhances the security up to a considerable level by generating random sequences but also provides a much faster rate of encryption and decryption [1], that's why LFSR based Unique Random Number Generator is chosen for the consideration. This paper deals in with the security issues of the new proposed system. Various types of cryptography attacks have been taken under consideration for original Playfair cipher but not vulnerable for this proposed cipher.

Playfair cipher Random number LFSR Polyalphabetic cipher