program to calculate the multiplicative inverse in Rijndal's Galois Feild.

rijndal_field_mul_inv.c

#include <stdio.h>

/* program to demonstrate how to find multiplicative inverse in Rijndal's Galois Field
 * part of the AES encryption algorithm or Rijndal's block cipher.
 *
 * code expanded on blog @ www.samiam/galois.html
 */


/* global vars */

unsigned char anti_log[256];
unsigned char glog[256];

/* functions */

unsigned char
gmul(unsigned char a, unsigned char b) {
        unsigned char p = 0;
        unsigned char counter;
        unsigned char hi_bit_set;

        for (counter = 0;counter < 8;counter++) {
                if ((b & 1) == 1)
                        p ^= a;
                hi_bit_set = (a & 0x80);
                a <<= 1;
                if (hi_bit_set == 0x80)
                        a ^= 0x1b;
                b >>= 1;
        }
        return p;
}

int
init() {
        int i;
        unsigned char p;
        anti_log[0]=0;
        p=1;

        printf("\n");
        for (i = 1;i < 256; i++) {
                p = gmul(p, 0xe5);
                anti_log[i] = p;
        }
        for (i = 1;i < 257; i++) {

                printf("%02x ",anti_log[i-1]);
                if (i % 16 == 0)
                        printf("\n");
        }
        printf("\n");
        printf("*** log table ***\n");
        /* now map log table */
        unsigned char temp;
        glog[0] = 0;
        glog[1] = 1;

        for (i = 1;i < 256;i++) {
                temp = anti_log[i];
                glog[temp]=i;
        }
        glog[1]=0x00;
        printf("\n");
        for (i = 1;i < 257;i++) {
                printf("%02x ",glog[i-1]);
                if (i % 16 == 0)
                        printf("\n");
        }
        printf("\n");

}

/* main entry */
                int
main() {
        init();

        /* try and multiply with log tables */
        int a;
        unsigned char result;
        printf("Enter number to calculate multiplicative inverse <0x00 - 0xff>? ");
        if (scanf("%x", &a) < 0) {
                printf("\nincorrect usage\n");
                return 1;
        }
        if (a > 255 || a < 0) {
                printf("number between 0-ff please!\n");
                return 1;
        }
        if (a == 0) {
                result = 0;
        } else {
                result = anti_log[(255 - glog[a])];
        }
        printf("result = %x\n", result);
}