### On two windows multivariate cryptosystem depending on random parameters

#### Abstract

The concept of multivariate bijective map of an affine space \(K^n\) over commutative Ring \(K\) was already used in Cryptography. We consider the idea of nonbijective multivariate polynomial map \(F_n\) of \(K^n\) into \(K^n\) represented as ''partially invertible decomposition'' \(F^{(1)}_nF^{(2)}_n \dots F^{(k)}_n\), \(k=k(n)\), such that knowledge on the decomposition and given value \(u=F(v)\) allow to restore a special part \(v'\) of reimage \(v\). We combine an idea of ''oil and vinegar signatures cryptosystem'' with the idea of linguistic graph based map with partially invertible decomposition to introduce a new cryptosystem. The decomposition will be induced by pseudorandom walk on the linguistic graph and its special quotient (homomorphic image). We estimate the complexity of such general algorithm in case of special family of graphs with quotients, where both graphs form known families of Extremal Graph Theory. The map created by key holder (Alice) corresponds to pseudorandom sequence of ring elements. The postquantum version of the algorithm can be obtained simply by the usage of random strings instead of pseudorandom.

#### Keywords

Cryptosystem, Multivariate cryptography, Postquantum cryptography, Algebraic incidence structure, Pseudorandom sequences, Pseudorandom walk in graph

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