In this thesis, the problem of a solvability of quadratic equation in a residue ring is presented. Legendre symbol is defined, which indices the solvability of a certain quadratic equation. This symbol is used in the formulation of the quadratic reciprocity law and its two supplements. These theorems are proved with the use of primitive nth roots of unity and the Gauss sums, and are used for specifying the value of the Legendre symbol. With the use of the results in this thesis the use of Goldwasser-Micali cryptosystem is shown.
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