International Association for Cryptologic Research

International Association
for Cryptologic Research

CryptoDB

Sugata Gangopadhyay

Publications

Year
Venue
Title
2015
EPRINT
2014
EPRINT
2010
EPRINT
On lower bounds of second-order nonlinearities of cubic bent functions constructed by concatenating Gold functions
Ruchi Gode Sugata Gangopadhyay
In this paper we consider cubic bent functions obtained by Leander and McGuire (J. Comb. Th. Series A, 116 (2009) 960-970) which are concatenations of quadratic Gold functions. A lower bound of second-order nonlinearities of these functions is obtained. This bound is compared with the lower bounds of second-order nonlinearities obtained for functions belonging to some other classes of functions which are recently studied.
2010
EPRINT
On second-order nonlinearities of some $\mathcal{D}_0$ type bent functions
Sugata Gangopadhyay Brajesh Kumar Singh
In this paper we study the lower bounds of second-order nonlinearities of bent functions constructed by modifying certain cubic Maiorana-McFarland type bent functions.
2009
EPRINT
On the Lower Bounds of the Second Order Nonlinearity of some Boolean Functions
Sugata Gangopadhyay Sumanta Sarkar Ruchi Telang
The $r$-th order nonlinearity of a Boolean function is an important cryptographic criterion in analyzing the security of stream as well as block ciphers. It is also important in coding theory as it is related to the covering radius of the Reed-Muller code $\mathcal{R}(r, n)$. In this paper we deduce the lower bounds of the second order nonlinearity of the two classes of Boolean functions of the form \begin{enumerate} \item $f_{\lambda}(x) = Tr_1^n(\lambda x^{d})$ with $d=2^{2r}+2^{r}+1$ and $\lambda \in \mathbb{F}_{2^{n}}$ where $n = 6r$. \item $f(x,y)=Tr_1^t(xy^{2^{i}+1})$ where $x,y \in \mathbb{F}_{2^{t}}, n = 2t, n \ge 6$ and $i$ is an integer such that $1\le i < t$, $\gcd(2^t-1, 2^i+1) = 1$. \end{enumerate} For some $\lambda$, the first class gives bent functions whereas Boolean functions of the second class are all bent, i.e., they achieve optimum first order nonlinearity.
2008
EPRINT
On Kasami Bent Functions
Deepmala Sharma Sugata Gangopadhyay
It is proved that no non-quadratic Kasami bent is affine equivalent to Maiorana-MacFarland type bent functions.
2007
EPRINT
On bent functions with zero second derivatives
Sugata Gangopadhyay
It is proved that a bent function has zero second derivative with respect to $a$, $b$, $a \ne b$, if and only if it is affine on all the flats parallel to the two dimensional subspace $V = \langle a, b \rangle$.
2006
EPRINT
On construction of non-normal Boolean functions
Sugata Gangopadhyay Deepmala Sharma
Given two non-weakly $k$-normal Boolean functions on $n$ variables a method is proposed to construct a non-weakly $(k+1)$-normal Boolean function on $(n+2)$ variables.
2006
EPRINT
On a new invariant of Boolean functions
Sugata Gangopadhyay Deepmala Sharma
A new invariant of the set of $n$-variable Boolean functions with respect to the action of $AGL(n,2)$ is studied. Application of this invariant to prove affine nonequivalence of two Boolean functions is outlined. The value of this invariant is computed for $PS_{ap}$ type bent functions.
2004
EPRINT
Crosscorrelation Spectra of Dillon and Patterson-Wiedemann type Boolean Functions
Sugata Gangopadhyay Subhamoy Maitra
In this paper we study the additive crosscorrelation spectra between two Boolean functions whose supports are union of certain cosets. These functions on even number of input variables have been introduced by Dillon and we refer to them as Dillon type functions. Our general result shows that the crosscorrelation spectra between any two Dillon type functions are at most $5$-valued. As a consequence we find that the crosscorrelation spectra between two Dillon type bent functions on $n$-variables are at most $3$-valued with maximum possible absolute value at the nonzero points being $\leq 2^{\frac{n}{2}+1}$. Moreover, in the same line, the autocorrelation spectra of Dillon type bent functions at different decimations is studied. Further we demonstrate that these results can be used to show the existence of a class of polynomials for which the absolute value of the Weil sum has a sharper upper bound than the Weil bound. Patterson and Wiedemann extended the idea of Dillon for functions on odd number of variables. We study the crosscorrelation spectra between two such functions and then use the results for the calculating the autocorrelation spectra too.
2003
EPRINT
Direct Sum of Non Normal and Normal Bent Functions Always Produces Non Normal Bent Functions
Sugata Gangopadhyay Subhamoy Maitra
Prof. Claude Carlet has pointed out an error in Theorem 1 of the paper. The error could not be recovered for the time being. Thus the statement presented in the title of the paper is not proved.
2003
EPRINT
Patterson-Wiedemann Construction Revisited
S. Gangopadhyay P. H. Keskar S. Maitra
In 1983, Patterson and Wiedemann constructed Boolean functions on $n = 15$ input variables having nonlinearity strictly greater than $2^{n-1} - 2^{\frac{n-1}{2}}$. Construction of Boolean functions on odd number of variables with such high nonlinearity was not known earlier and also till date no other construction method of such functions are known. We note that the Patterson-Wiedemann construction can be understood in terms of interleaved sequences as introduced by Gong in 1995 and subsequently these functions can be described as repetitions of a particular binary string. As example we elaborate the cases for $n = 15, 21$. Under this framework, we map the problem of finding Patterson-Wiedemann functions into a problem of solving a system of linear inequalities over the set of integers and provide proper reasoning about the choice of the orbits. This, in turn, reduces the search space. Similar analysis also reduces the complexity of calculating autocorrelation and generalized nonlinearity for such functions. In an attempt to understand the above construction from the group theoretic view point, we characterize the group of all $GF(2)$-linear transformations of $GF(2^{ab})$ which acts on $PG(2,2^a)$.