On Welch-Gong Transformation Sequence Generators

We explore three applications of geometric sequences in constructing cryptographic Boolean functions. First, we construct 1-resilient functions of n Boolean variables with nonlinearity 2n-1-2(n-1)/2, n odd. The Hadamard transform of these functions is 3-valued, which limits the efficiency of certain stream cipher attacks. From the case for n odd, we construct highly nonlinear 1-resilient functions which disprove a conjecture of Pasalic and Johansson for n even. Our constructions do not have a potential weakness shared by resilient functions which are formed from concatenation of linear functions. Second, we give a new construction for balanced Boolean functions with high nonlinearity, exceeding 2n-1-2(n-1)/2, which is not based on the direct sum construction. Moreover, these functions have high algebraic degree and large linear span. Third, we construct balanced vectorial Boolean functions with nonlinearity 2n-1-2(n-1)/2 and low maximum correlation. They can be used as nonlinear combiners for stream cipher systems with high throughput.

The Welch-Gong (WG) stream cipher family was designed based on the WG transformation and is able to generate keystreams with mathematically proven randomness properties such as long period, balance, ideal tuple distribution, ideal two-level autocorrelation and high and exact linear complexity. In this paper, we present a compact hardware architecture and its pipelined implementation of the stream cipher WG-16, an efficient instance of the WG stream cipher family, using composite field arithmetic and a newly proposed property of the trace function in tower field representation. Instead of using the original binary field F2^16, we demonstrate that its isomorphic tower field F(((2^2)^2)^2)^2 can lead to a more efficient hardware implementation. Efficient conversion matrices connecting the binary field F2^16 and the tower field F(((2^2)^2)^2)^2 are also derived. Our implementation results show that the pipelined WG-16 hardware core can achieve the throughput of 124 MHz at the cost of 478 slices in an FPGA and 552 MHz at the cost of 12,031 GEs in a 65 nm ASIC, respectively.

Boolean functions are used as nonlinear filter functions and combiner functions in several stream ciphers. The security of these stream ciphers largely depends upon cryptographic properties of Boolean functions. Finding a balanced Boolean function with optimal cryptographic properties is an open research problem in the cryptographic community. Since the number of n-variable Boolean functions is $$2^{2^n}$$ , it is not computationally feasible to search the entire space of such functions for cryptographically significant functions when $$n \ge 6$$ . In general, the construction of Boolean functions with optimal or near optimal cryptographically significant properties is formulated as combinatorial optimization problems. In this paper, we apply the Genetic algorithm with the integration of a local search procedure called hybrid GA (HGA) searching for the Boolean functions with high nonlinearity and low autocorrelation. The performance of the Genetic algorithm depends upon the tuning parameters such as crossover mechanism, mutation probability, selection criteria, and the choice of fitness/cost function. To achieve an optimal trade-off among two cryptographic properties, the choice of the cost function plays an important role for finding an optimal solution. So our main focus is to construct balanced Boolean functions using HGA with a new cost function and compare it with existing cost functions. Our experimental results shows that the HGA is more efficient than GA and the new cost function is more efficient than existing cost functions for reaching the optimal solutions.

A general structure of the Welch-Gong (WG) stream cipher family is based on filtering an m-sequence of degree l over a finite field $\ensuremath{{\mathbb{F}}}_{2^m}$ where the filtering function is a WG transformation from $\ensuremath{{\mathbb{F}}}_{2^m}$ to $\ensuremath{{\mathbb{F}}}_{2}$ . For a fixed m and l, the linear span of the filtering sequence can be enhanced by increasing the algebraic degree of the WG transformations. This can be accomplished by the composition of a WG transformation with a monomial permutation, which is called the decimation of a WG transformation. In this paper, we first present the new exponent set of WG transformations, and show the existence of exponents derived from the new exponent set for which a decimated WG transformation achieves the maximum algebraic degree. As a result, the linear span of keystreams produced by a decimated WG cipher can be maximized and calculated theoretically. We then give a description of a decimated WG stream cipher which is built upon an LFSR and a decimated WG transformation over an extension field. The randomness properties of keystreams produced by a decimated WG cipher are derived based on the new exponent set. We also discuss the selection criteria for choosing the optimal parameters for the WG cipher family in order to achieve the maximum level of security. Finally, we present the optimal parameters for the WG transformations over $\ensuremath{{\mathbb{F}}}_{2^m}, 7\leq m \leq 16$ based on the proposed criteria.

Boolean functions play a central role in the security of stream and block ciphers (in symmetric cryptography). They are functions mapping binary vectors of some length to either bits (in which case we have Boolean functions strictly speaking) or to binary vectors (of the same length or of another length, we are then speaking of vectorial Boolean functions). In the first case, they play roles in stream ciphers and in the second case, they are mainly used as substitution boxes in block ciphers. Being in each case the only nonlinear components of the ciphers, they are essential to their security. They are also the main components of the ciphers whose choice induces the speed. A tradeoff must then be found between the security and the speed of the cipher. The AES winner (used nowadays as a NIST standard for civil encryption, in particular between all banks) has been chosen because it allowed the best tradeoff. We shall describe what are the main results on them and what are the main open problems they pose currently.

Boolean functions with maximum algebraic immunity have been considered as one class of cryptographically significant functions. It is known that Boolean functions on odd variables have maximum algebraic immunity if and only if a correlative matrix has column full rank, and Boolean functions on even variables have maximum algebraic immunity if and only if two correlative matrices have column full rank. Recently, a smaller matrix was used in the odd case. We find that one or two smaller matrices can be used in the even case and consequently present several sufficient and necessary conditions for Boolean functions with maximum algebraic immunity. This result advances the ability to identify whether Boolean functions on even variables achieve maximum algebraic immunity. We also present a construction algorithm for n-variable Boolean functions with maximum algebraic immunity, specially with the Hamming weights of \( \sum {_{i = 0}^{\left\lceil {\frac{n} {2}} \right\rceil - 1} } \left( {\begin{array}{*{20}c} n \\ i \\ \end{array} } \right) \). It is easily realized for not too large n and helps construct balanced Boolean functions with maximum algebraic immunity on even variables. Furthermore, we present a sufficient and necessary condition for balanced Boolean functions to achieve maximum algebraic immunity and optimum algebraic degree, and modify the construction algorithm to construct Boolean functions on odd variables with maximum algebraic immunity, optimum algebraic degree and high nonlinearity.

Boolean functions exhibiting strong cryptographic characteristics are essential crypto primitive to be included in the design of secure cryptosystems. It is not only common to incorporate Boolean functions in the design of symmetric block and stream ciphers but they also have a key place in the design of cryptographic hash functions. Strong Boolean functions make the system secure and resistant against cryptanalytic attacks. A wide range of approaches have been adopted in the discovery of Boolean functions that excel in terms of several cryptographic characteristics. In this paper, we present a new scheme based on Genetic Algorithm to generate Boolean functions which satisfy balancedness, correlation immunity, algebraic degree and nonlinearity characteristics. Proposed scheme generates strong Boolean functions with desired values of these characteristics.

We develop an improved cost function to be used in simulated annealing followed by hill-climbing to find Boolean functions satisfying multiple desirable criteria such as high nonlinearity, low autocorrelation, balancedness, and high algebraic degree. Using this cost function that does not necessitate experimental search for parameter tuning, the annealing-based algorithm reaches the desired function profiles more rapidly. Some Boolean functions of eight and nine variables have been found, which are unattained in the computer search based literature, in terms of joint optimization of nonlinearity and autocorrelation. Global characteristics of eight-variable Boolean functions generated by algebraic construction or computer search are compared with respect to the sum-of-squared-errors in their squared spectra, which is also proportional to the sum-of-squared-errors in their autocorrelation function, the term ‘error’ denoting the deviation from bent function characteristics. Preliminary results consisting of cryptographically strong Boolean functions of nine, ten and eleven variables obtained using a three-stage optimization technique are also presented.

Boolean functions play an important role in symmetric ciphers. One of important open problems on Boolean functions is determining the maximum possible resiliency order of n-variable Boolean functions with optimal algebraic immunity. In this letter, we search Boolean functions in the rotation symmetric class, and determine the maximum possible resiliency order of 9-variable Boolean functions with optimal algebraic immunity. Moreover, the maximum possible nonlinearity of 9-variable rotation symmetric Boolean functions with optimal algebraic immunity-resiliency trade-off is determined to be 224.

The need for stream ciphers in communication and information cryptographic systems is rapidly grown in the last twenty years. The stream ciphers usually use a pseudo-random number generator with large period, large linear complexity and good statistical properties. At present a small number of software-oriented stream ciphers are known. Due to this reasons in our paper we suggest a new pseudo random number generator, named P-adic shrinking-multiplexing generator, and give its statistical analysis.

The Boolean functions satisfying secure properties on the restricted sets of inputs are studied recently due to their importance in the framework of the FLIP stream cipher. However, finding Boolean functions with optimal cryptographic properties is an open research problem in the cryptographic community. This paper presents an Improved Genetic Algorithm (IGA) with the directed changes that keep the weightwise balancedness of Boolean functions. A cross-protection strategy is proposed to ensure that the offspring has the same weightwise balancedness characteristics of the parents while implementing crossover. Then, a large number of weightwise (almost) perfectly balanced (W(A)PB) functions with a good nonlinearity profile are obtained based on IGA. Finally, we make comparisons between our constructions and relevant works. The comparisons show that IGA has a significant advantage for reaching the W(A)PB functions with high weightwise nonlinearity. Moreover, it is the first time to obtain the 8-variable WPB functions with the weightwise nonlinearity of 28 in the restricted sets of inputs with Hamming weight of 4, and list the statistical indicators of the weightwise nonlinearity for W(A)PB functions for input size n = 9, 10.

The elementary cellular automata (ECAs) under the chaotic rule possess long periodicity and are widely used in pseudo-random number generators. However, their period is limited, related to the rule and the number of cells. Meanwhile, the Boolean functions of some ECAs are linear and vulnerable to linear analysis. Thus, the ECA cannot be directly implemented in the stream cipher. In this paper, a hybrid ECA (HECA) with dynamic mask (HECA-M) is designed. The HECA-M consists of two parts: the driving and mask parts. The driving part based on a HECA is used in generating the keystream, and the mask part based on a chaotic ECA is utilized to determine the iterative rule of the driving part. Subsequently, a stream cipher based on the HECA-M and SHA-512 is proposed. The statistic and secure analyses indicate that the proposed stream cipher possesses good randomness and can resist stream cipher analyses, such as exhaustive search, Berlekamp–Massey synthesis, guess and determine attack, time–memory–data tradeoff attack, etc. Hence, the proposed scheme can meet security requirements. Moreover, the time and space consumption of the proposed stream cipher is qualified.

We improve the steepest descent algorithm and increase the double threshold parameter, which significantly improves the algorithm’s efficiency. And we design a new cost function so that in terms of search, various characteristics of Boolean functions can be taken into account simultaneously. Applying our algorithm, there are excellent results regarding the 9, 10, 11, and 12 variables. We find a Boolean function with a nonlinearity of 242 in 9 variables and the whole search space. Previously, this result only appeared in the rotational symmetry class. The best-achieved nonlinearity result for permutation (6, 5, 1, 4, 7, 2, 3, 0, 8) and (0, 7, 2, 5, 4, 1, 3, 6, 8) class is 238 and 239 introduced by Kavut in Information and Computation (2010). Still, applying our algorithm, we obtain a balanced Boolean function with a nonlinearity of 240 under the same permutation, indicating that our method is more general. Among the 11 variables, a Boolean function with a higher nonlinearity and a lower transparency level and the absolute value spectrum are maintained at a lower level. The algorithm performs well when considering all aspects of the property. There are similarly promising results in even-numbered variables.

Boolean functions have found a widespread use in cryptography. In connection with the advent of the 'algebraic' attack on stream ciphers, the Boolean functions, which are used in these ciphers as nonlinear filters, have to possess, among other properties, high algebraic immunity. One more cryptographically significant property of Boolean functions, especially of those utilised in stream ciphers, is nonlinearity. In this connection, the question arises about relations between the nonlinearity of a Boolean function and its algebraic immunity.

Nowadays, the resistance against algebraic attacks and fast algebraic attacks are considered as an important cryptographic property for Boolean functions used in stream ciphers. Both attacks are very powerful analysis concepts and can be applied to symmetric cryptographic algorithms used in stream ciphers. The notion of algebraic immunity has received wide attention since it is a powerful tool to measure the resistance of a Boolean function to standard algebraic attacks. Nevertheless, an algebraic tool to handle the resistance to fast algebraic attacks is not clearly identified in the literature. In the current paper, we propose a new parameter to measure a Boolean function's resistance to fast algebraic attack. We also introduce the notion of fast immunity profile and show that it informs both on the resistance to standard and fast algebraic attacks. Further, we evaluate our parameter for two secondary constructions of Boolean functions. Moreover, A coding-theory approach to the characterization of perfect algebraic immune functions is presented. Via this characterization, infinite families of binary linear complementary dual codes (or LCD codes for short) are obtained from perfect algebraic immune functions. Some of the binary LCD codes presented in this paper are optimal. These binary LCD codes have applications in armoring implementations against so-called side-channel attacks (SCA) and fault non-invasive attacks, in addition to their applications in communication and data storage systems.