Correlation attacks using a new class of weak feedback polynomials

In English In 1985 Siegenthaler introduced the concept of correlation attacks on LFSR based stream ciphers. A few years later Meier and Staffelbach demonstrated a special technique, usually referred to as fast correlation attacks, that is very effective if the feedback polynomial has a special form, namely, if its weight is very low. Due to this seminal result, it is a well known fact that one avoids low weight feedback polynomials in the design of LFSR based stream ciphers. This paper identifies a new class of such weak feedback polynomials, polynomials of the form f(x) = g(1) (x) + g(2) (x)x(M1) + (...) + g(t)(x)x(Mt-1), where g(1), g(2), (...), g(t) are all polynomials of low degree. For such feedback polynomials, we identify an efficient correlation attack in the form of a distinguishing attack.