A method, is provided for efficiently determining an inverse multiplicative modulo. In many public-key cryptographic algorithms, an inverse modulo is usually calculated in key generation. However, because many Reduced Instruction Set Computer (RISC) processors do not have the hardware support for division, good results are often not yielded. Therefore, to efficiently calculate hail an inverse modulo, a modified algorithm that utilizes a minimum of three division and two multiplications in conjunction with shifts and addition/subtractions is employed. The modified algorithm then is able to efficiently utilize the properties of the RISC processors to yield good results, especially when developing keys for public-key cryptographic algorithms.