Department of Statistics Seminar
North Carolina State University

presents

Dr. Bimal Roy

Indian Statistical Institute

"Cryptanalysis of LFSR based Cryptosystems when LFSR's are long & the combining function is unknown"

ABSTRACT

A commonly used cryptosystem is based on LFSR(linear feedback shift register)'s. Output of several LFSR's are combined thru a boolean function to produce the "key-stream" which is then bit-wise added( modulo 2) to the binary message text to get the ciphertext. This ciphertext is communicated. We assume that the LFSR's are long,say 128 bits & the combining function is unknown. From a ciphertext we estimate the seeds of all the LFSR's and the combining function. The method used is statistical; to be specific, we use posterior probabilities & maximum likelihood method.

(In my previous talk in 1998, I considered the same problem but assumed that the LFSR's are at most 32-bit long & the combining function is known; we estimated the seeds of all the LFSR's).

Friday, May 26, 2000

10:30 - 11:30

2 Patterson Hall

Refreshments will be served.