Apparatus and method for ciphertext comparison capable of preventing side channel attack

Abstract

A ciphertext comparison method according to an embodiment is performed by a processor in a computing apparatus, and the method includes an operation of segmenting a first ciphertext and a second ciphertext into m part bitstreams (in this instance, m is a natural number satisfying 1<m), respectively; an operation of extracting a value corresponding to a j−1part bitstream (in this instance, j=i+1, i is a natural number satisfying 0≤i≤m−1) of the first ciphertext and a j−1part bitstream of the second ciphertext, as a jintermediate value between a first value and a second value in a first lookup table including the first value and the second value of which Hamming weights are identical; an operation extracting a value corresponding to the jintermediate value and a j−1result value, as a jresult value between a third value and a fourth value in a second lookup table including the third value and the fourth value of which Hamming weights are identical; and in a case of j≠m, an operation of repeatedly performing extraction as the jintermediate value and extraction as the jresult value by increasing J; and in a case of j=m, an operation of outputting an mresult value as a result value of comparison between the first ciphertext and the second ciphertext.