Patent ID: 7337203

Claim:
An exponent calculation apparatus for encrypting or decrypting data by calculating x e based on two integers x and e, the apparatus comprising: an input unit for inputting the two integers x and e; a candidate exponents storing unit for storing candidate exponents {l_i} (0≦i≦L−1), the number of the candidate exponents being L; a pre-calculation unit for pre-calculating x^{l_i} for each of the candidate exponents {l_i}, which are stored in the candidate exponents storing unit, based on the input integer x; a pre-calculated values storing unit for storing the values x^{l_i} obtained by the pre-calculation; a dividing unit for dividing the input integer e into a plurality of values {f_i} (0≦i≦F−1) so that each of the values {f_i} corresponds to one of the candidate exponents {l_i}; a calculation result storing unit for storing a calculation result c; a sequential processing unit for sequentially updating the calculation result c for each of the divided values {f_i} (0≦i≦F−1) by using each of the pre-calculated values x^{l_i}; and an output unit for outputting the calculation result c for encrypting or decrypting data after the calculation result c is updated for all the values {f_i} as x e .