A method for preventing side-channel attack according to an embodiment includes generating an order table which includes a position index value for each bit value of a bit string that is secret information to be generated through a decryption algorithm of an Nth Degree Truncated Polynomial Ring Units (NTRU) LPRime algorithm, shuffling a sort order of the position index value for the each bit value in the order table based on a random number, determining a generation order for the each bit value according to the sort order of the position index value for the each bit value in the order table, and generating the secret information through the decryption algorithm. The secret information is generated by generating the each bit value according to the determined generation order.