Weak points in book cipher system


This paper includes determining the weak points of cryptosystem that use the letters of a certain book as key in encryption and decryption processes. It depends on study the letters frequency distribution of cipher text and the characteristics of the language that is used in writing the book rather than the language of the plain text. It takes in consideration, using English language in writing the plain text, key, and cipher text. The research finds much vulnerability that can be explored to reduce the key space and reduce the number of possibilities when attack the encrypted message in this system. For example, there is ability to reduce the possibility number of key letter of for the first three columns from 17576 to 2475 with percentage 90.9%. And the relation between columns letters reduced between 50% and 73%. Besides, the possibilities of the plain letters for each cipher text letter can reduced with 50%.