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

A ciphertext comparison apparatus, the apparatus comprising:

• a memory storing one or more instructions;
• and
• one or more processors executing the one or more instructions,
• wherein the one or more processors are configured to:
• segment a first ciphertext and a second ciphertext into m part bitstreams (in this instance, m is a natural number satisfying 1<m), respectively;
• extract a value corresponding to a j−1th part bitstream (in this instance, j=i+1, i is a natural number satisfying 0≤i≤m−1) of the first ciphertext and a j−1th part bitstream of the second ciphertext, as a jth intermediate 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;
• extract a value corresponding to the jth intermediate value and a j−1th result value, as a jth result 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;
• in a case of j≠m, repeatedly perform extraction as the jth intermediate value and extraction as the jth result value by increasing J;
• and
• in a case of j=m, output an mth result value as a result value of comparison between the first ciphertext and the second ciphertext.

A non-transitory computer-readable storage medium storing instructions which, when executed by a processor, cause a computing apparatus comprising the processor to perform operations for text-based document classification, the operations comprising:

• segmenting a first ciphertext and a second ciphertext into m part bitstreams (in this instance, m is a natural number satisfying 1<m), respectively;
• extracting a value corresponding to a j−1th part bitstream (in this instance, j=i+1, i is a natural number satisfying 0≤i≤m−1) of the first ciphertext and a j−1th part bitstream of the second ciphertext, as a jth intermediate 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;
• extracting a value corresponding to the jth intermediate value and a j−1th result value, as a jth result 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, repeatedly performing extraction as the jth intermediate value and extraction as the jth result value by increasing J;
• and
• in a case of j=m, outputting an mth result value as a result value of comparison between the first ciphertext and the second ciphertext.