Wilfried Meidl

On the expected value of the linear complexity and the k-error linear complexity of periodic sequences

Rueppel (1986) conjectured that periodic binary sequences have expected linear complexity close to the period length N. In this paper, we determine the expected value of the linear complexity of N-periodic sequences explicitly and confirm Rueppels conjecture for arbitrary finite fields. Cryptographically strong sequences should not only have a large linear complexity, but also the change of a few terms should not cause a significant decrease of the linear complexity. This requirement leads to the concept of the k-error linear complexity of N-periodic sequences. We present a method to establish a lower bound on the expected k-error linear complexity of N-periodic sequences based on the knowledge of the counting function /spl Nscr//sub N/,/sub 0/(c), i.e., the number of N-periodic sequences with given linear complexity c. For some cases, we give explicit formulas for that lower bound and we also determine /spl Nscr//sub N,0/(c).

Linear Complexity, k-Error Linear Complexity, and the Discrete Fourier Transform

Complexity measures for sequences of elements of a finite field play an important role in cryptology. We focus first on the linear complexity of periodic sequences. By means of the discrete Fourier transform, we determine the number of periodic sequences S with given prime period length N and linear complexity LN, 0(S)=c as well as the expected value of the linear complexity of N-periodic sequences. Cryptographically strong sequences should not only have a large linear complexity, but also the change of a few terms should not cause a significant decrease of the linear complexity. This requirement leads to the concept of the k-error linear complexity LN, k(S) of sequences S with period length N. For some k and c we determine the number of periodic sequences S with given period length N and LN, k(S)=c. For prime N we establish a lower bound on the expected value of the k-error linear complexity.

The expected value of the joint linear complexity of periodic multisequences

Complexity measures for sequences of elements of a finite field, such as the linear complexity, play an important role in cryptology. Recent developments in stream ciphers point towards an interest in word-based (or vectorized) stream ciphers, which require the study of the complexity of multisequences. We extend a well-known relationship between the linear complexity of an N-periodic sequence and the (generalized) discrete Fourier transform of N- tuples to the case of multisequences. Using the concept of the generalized discrete Fourier transform for multisequences, we compute the expected value of the joint linear complexity of random periodic multisequences, and for some types of period lengths N we determine the number NNt(c) of t N-periodic sequences with given joint linear complexity c.

On the stability of 2/sup n/-periodic binary sequences

The k-error linear complexity of a periodic binary sequence is defined to be the smallest linear complexity that can be obtained by changing k or fewer bits per period. This contribution focuses on the case of 2n-periodic binary sequences. For k=1,2, the exact formula for the expected k-error linear complexity of a sequence having maximal possible linear complexity 2n, and the exact formula of the expected 1-error linear complexity of a random 2n-periodic binary sequence are provided. For k ges 2, lower and upper bounds on the expected value of the k-error linear complexity of a random 2n-periodic binary sequence are established

How Many Bits have to be Changed to Decrease the Linear Complexity

The k-error linear complexity of periodic binary sequences is defined to be the smallest linear complexity that can be obtained by changing k or fewer bits of the sequence per period. For the period length pn, where p is an odd prime and 2 is a primitive root modulo p2, we show a relationship between the linear complexity and the minimum value k for which the k-error linear complexity is strictly less than the linear complexity. Moreover, we describe an algorithm to determine the k-error linear complexity of a given pn-periodic binary sequence.

Error linear complexity measures for multisequences

Complexity measures for sequences over finite fields, such as the linear complexity and the k-error linear complexity, play an important role in cryptology. Recent developments in stream ciphers point towards an interest in word-based stream ciphers, which require the study of the complexity of multisequences. We introduce various options for error linear complexity measures for multisequences. For finite multisequences as well as for periodic multisequences with prime period, we present formulas for the number of multisequences with given error linear complexity for several cases, and we present lower bounds for the expected error linear complexity.

A construction of bent functions from plateaued functions

In this presentation, a technique for constructing bent functions from plateaued functions is introduced and analyzed. This generalizes earlier techniques for constructing bent from near-bent functions. Using this construction, we obtain a big variety of inequivalent bent functions, some weakly regular and some non-weakly regular. Classes of bent functions having some additional properties that enable the construction of strongly regular graphs are formed, and explicit expressions for bent functions with maximal degree are presented.

A construction of weakly and non-weakly regular bent functions

In this article a technique for constructing p-ary bent functions from near-bent functions is presented. This technique is then used to obtain both weakly regular and non-weakly regular bent functions. In particular we present the first known infinite class of non-weakly regular bent functions.

On the linear complexity profile of explicit nonlinear pseudorandom numbers

Bounds on the linear complexity profile of a general explicit nonlinear pseudorandom number generator are obtained. For some special explicit nonlinear generators including the explicit inversive generator these results are improved.

Some Notes on the Linear Complexity of Sidel'nikov-Lempel-Cohn-Eastman Sequences

We continue the study of the linear complexity of binary sequences, independently introduced by Sidel’nikov and Lempel, Cohn, and Eastman. These investigations were originated by Helleseth and Yang and extended by Kyureghyan and Pott. We determine the exact linear complexity of several families of these sequences using well-known results on cyclotomic numbers. Moreover, we prove a general lower bound on the linear complexity profile for all of these sequences.