Patent ID: 8638927
Filing Date: 2014-01-28
Classification: G06F,H04L

Abstract:
1. A cryptographic processing method that repeatedly executes a modular-multiplication operation in which multiplication and a remainder operation are performed with respect to a first input and a second input so as to perform a modular-exponentiation operation that is expressed by: where C represents an input value, d represents key data, M represents output data, and mod N represents a remainder operation based on modulus N so as to perform encryption and decryption, the method comprising: inputting random number data and the key data d, and of calculating key data d′ by subtracting the random number data r from the key data d; calculating values −C and −C executing the modular-multiplication operation using a result of a previously repeated operation as the first input and the second input in a first operation cycle which is sequentially and repeatedly executed starting from significant bits corresponding to respective bit values d wherein in the second operation cycle in the repeated modular-multiplication operation: skipping the modular-multiplication operation when the bit value d′ reading a value of the stored value −C to use the value as the second input, using a result of the previously repeated operation as the first input, and executing the modular-multiplication operation when the bit value d′ reading a value of the stored value C to use the value as the second input, using a result of the previously repeated operation as the first input, and executing the modular-multiplication operation when the bit value d′ reading a value of the stored value −C