US Patent 7206799 Modular multiplication method and calculating device

To provide a modular multiplication method and a calculating device that do not rely on the Montgomery technique, wherein the number of times of multiply-add calculations is reduced to shorten a calculation time for calculation speed-up, there is no limitation in input value, and it is possible to execute a remainder calculation exceeding the calculable maximum bit length of a multiply-add unit that is used. Assuming that N=2n−M and X=α×2n+β, a relation of XmodN=(α×M+β)modN is derived, which is utilized. n represents a maximum bit number where “1” is assigned in N, a solution of 2n+1modN is set as b, A×B is set as X, XmodN is transferred to (X/2n+1×b+Xmod2n+1)modN and further transferred to (X·n/2n+1×b+X·nmod2n+1)modN, calculations of X·n/2n+1×b+X·nmod2n+1 are repeated until a bit length of X·n becomes n+1, X·n−N is derived and a derived result is set as a solution of “A×BmodN”.