Electronic apparatus, control method thereof and server

An electronic apparatus comprising:

• a memory storing a composite function in which at least two polynomials are composed;
• and
• a processor configured to, based on a comparison operation command being received for a plurality of homomorphic ciphertexts, perform operation by reflecting the plurality of homomorphic ciphertexts to the composite function, and obtain a comparison result of the plurality of homomorphic ciphertexts based on the operation result,
• wherein each of the at least two polynomials outputs a value in a preset range for a value in a preset domain, and
• wherein a domain of one of the at least two polynomials is determined based on a range of a previous polynomials;
• wherein the memory stores a first polynomial and a second polynomial forming the composite function,
• wherein the processor is further configured to:
• obtain a first operation result by reflectinq the plurality of homomorphic ciphertexts to the first polynomial, obtain a second operation result by reflecting the first operation result to the second polynomial, and obtain a comparison result of the plurality of homomorphic ciphertexts based on the second operation result,
• wherein the first polynomial outputs a value in a first range for a value within a first domain,
• wherein the second polynomial outputs a value in a second range for a value in a second domain, and
• wherein the second domain is determined based on the first range.