INFORMATION THEORETIC APPROACH TO SECURE LSFR CIPHERS

To break a normal LFSR cipher, a cryptanalyst needs only 2n bits of corresponding plain and ciphertext, where n is the number of stages of the shift register.l] In this paper, a method of substituting completely random characters into the ciphertext and therefore preventing the encipherment of a full 2n-length sequence (under its proper key) will be discussed. Due to the high redundancy of English, a cipher containing several completely random characters will still be readable.