A second-order differential power analysis attack method and a second-order differential electromagnetic analysis attack method are provided to be applied to an encoding apparatus in which a plurality of intermediate values is parallel processed. A power consumption of an encoding apparatus is measured(301). The measured power consumption is processed by using a pre-process function(302). A plurality of intermediate values about each presumed key is calculated by inputting a plurality of presumed keys to an estimated model(303). A power consumption corresponding to each presumed key is estimated by using the intermediate values. A correlation coefficient between the pre-processed power consumption and the estimated power consumption is calculated(304). A key used in the encoding apparatus among the presumed keys is determined by using the correlation coefficient(305).
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