Rijndail S-box the long way

rijndael_field_sbox.c

#include <stdio.h>

/* Rijndael Field S-box transformation
 * part of the AES encryption algorythm 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 GF(256) <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;
        }

//      a = 83; /* 0x53 */

        if (a == 0) {
                result = 0;
        } else {
                result = anti_log[(255 - glog[a])];
        }
        printf("multiplicitave inverse = %x\n", result);
/* affine transformation the long way */
        int j, i;
        unsigned char r[8], p, m = 248, af = 0;
        r[0] = 0;

        for (j = 7;j >= 0;j--) {
                r[j] = 0;
                p = m & result;
                for (i = 0;i < 8;i++) {
                        r[j] ^= ((p >> i) & 0x01);
                }
                m = (m >> 1) | (m << 7); /* rotate right 1 */
        }

        for (i = 0;i < 8;i++) {
                af = af | (r[i] << i);
        }
        af ^= 99; /* 0x63 */

        printf("affine trans = %x\n", af);
}