Çağdaş Çalık

Message recovery and pseudo-preimage attacks on the compression function of Hamsi-256

Hamsi is one of the second round candidates of the SHA-3 competition. In this study, we present non-random differential properties for the compression function of Hamsi-256. Based on these properties, we first demonstrate a distinguishing attack that requires a few evaluations of the compression function. Then, we present a message recovery attack with a complexity of 210.48 compression function evaluations. Also, we present a pseudo-preimage attack for the compression function with complexity 2254.25.

New Distinguishers Based on Random Mappings against Stream Ciphers

Statistical randomness testing plays an important role in security analysis of cryptosystems. In this study, we aim to propose a new framework of randomness testing based on random mappings. Considering the probability distributions of coverage and ρ-lengths, we present three new distinguishers; (i) coverage test, (ii) ρ-test and (iii) DP-coverage test and applied them on Phase III Candidates of eSTREAM project. We experimentally observed some statistical weaknesses of Pomaranch using the coverage test.

Computing the weight of a boolean function from its algebraic normal form

We present an algorithm that computes the weight of a Boolean function from its Algebraic Normal Form (ANF). For functions acting on high number of variables (n>30) and having low number of monomials in its ANF, the algorithm is advantageous over the standard method of computing weight which requires the transformation of functions ANF to its truth table with a complexity of

Counting Boolean functions with specified values in their Walsh spectrum

\mathcal{O}(n2^n)

On Feedback Functions of Maximum Length Nonlinear Feedback Shift Registers

operations. A relevant attempt at computing the Walsh coefficients of a function from its ANF by Gupta and Sarkar required the function to be composed of high degree monomials [1]. The proposed algorithm overcomes this limitation for particular values of n, enabling the weight and Walsh coefficient computation for functions that could be of more interest for practical applications.

The Multiplicative Complexity of 6-variable Boolean Functions.

The problem of counting Boolean functions with specified number s of Walsh coefficients @w in their Walsh spectrum is discussed in this paper. Strategies to solve this problem shall help solving many more problems related to desired cryptographic features of Boolean functions such as nonlinearity, resiliency, algebraic immunity, etc. In an attempt to study this problem, we present a new framework of solutions. We give results for |@w|>=2^n^-^1 and for all s, in line with a previous work of Wu (1998) [12]. We also provide various results such as existence and construction for some s when @w=0, multiplicities for all @w and naive bounds on s for @w>2^n^/^2.