The notion of &rdquo / Planar functions&rdquo / goes back to Dembowski and Ostrom, who introduced it in 1968 first time to describe projective planes with special properties in finite geometry. Recently, they attracted an interest from cryptography because of having an optimal resistance to differential cryptanalysis.This thesis is based on the paper &rdquo / New semifields, PN and APN functions&rdquo / by J&uuml / rgen Bierbrauer. The whole purpose of this thesis is to understand and present a detailed description of the results of the paper of Bierbrauer about planar functions. Here and throughout this thesis &rdquo / new&rdquo / means &rdquo / new&rdquo / in the paper of Bierbrauer. In particular we have no new constructions here and we only explain the results of Bierbrauer.

The aim of this diploma thesis is to introduce the topic of semifields and to explain its connection with planar functions. From its beginning the thesis leads to the formulation of relation between commutative se- mifields of odd order and planar Dembowski-Ostrom polynomials, which R. S. Coulter and M. Henderson introduce in their article from 2008. At the beginning of the thesis there is a short introduction to projective and affine planes. The thesis further describes coordinatization of projective plane by planar ternary ring. It also aims to investigate properties of ternary ring depending on the number of perspectivities in the projective plane. One of the chapters is dedicated to the isotopy of loops, which can be applied directly on the isotopy of semifields. The thesis mainly focuses on the proof of denoted correspondence between commutative semifields of odd order and planar Dembowski-Ostrom polynomials. Finally, several corrolaries of this relation and the isotopy of semifields are declared. 1

Let Fqt be the finite field with qt elements and let F*qt be its multiplicative group. We study the diagonal equation axq−1 + byq−1 = c, where a,b and c ∈ F*qt. This equation can be written as xq−1+αyq−1 = β, where α, β ∈ F ∗ q t . Let Nt(α, β) denote the number of solutions (x,y) ∈ F*qt × F*qt of xq−1 + αyq−1 = β and I(r; a, b) be the number of monic irreducible polynomials f ∈ Fq[x] of degree r with f(0) = a and f(1) = b. We show that Nt(α, β) can be expressed in terms of I(r; a, b), where r | t and a, b ∈ F*q are related to α and β. A recursive formula for I(r; a, b) will be given and we illustrate this by computing I(r; a, b) for 2 ≤ r ≤ 4. We also show that N3(α, β) can be expressed in terms of the number of monic irreducible cubic polynomials over Fq with prescribed trace and norm. Consequently, N3(α, β) can be expressed in terms of the number of rational points on a certain elliptic curve. We give a proof that given any a, b ∈ F*q and integer r ≥ 3, there always exists a monic irreducible polynomial f ∈ Fq[x] of degree r such that f(0) = a and f(1) = b. We also use the result on N2(α, β) to construct a new family of planar functions.