APPARATUS FOR PERFORMING ARITHMETIC OPERATION USING SECRET INFORMATION, METHOD AND PROGRAM

PROBLEM TO BE SOLVED: To provide a decoding apparatus which improves security of arithmetic processing using secret key. SOLUTION: The decoding apparatus decodes cipher text data obtained by ciphering plain text data by a cipher system based on a discrete logarithm problem on a subgroup of a multiplicative group, and is provided with: an input part 101 which inputs the cipher text data which is an element on the subgroup and represented by affine representation; a conversion part 111 which selects any one of a plurality of maps for converting affine representation into projection representation at random and converts the input cipher text data into the projection representation by the selected map; and an arithmetic processing part 112 which calculates the plain text data by executing decoding processing which is predetermined by the cipher system to the cipher text data converted into the projection representation. COPYRIGHT: (C)2010,JPO&INPIT