c algorithm to decipher affine cipher

aff_c.c

#include <stdio.h>
#include <stdlib.h>

/* quick program to solve affine cipher with key a, b mod 26 */
/* usage: ./aff_c <key a> <key b> <optional e=encrypt> 
 *        cipher-text from stdin or pipe
 */


int
main(int argc, char *argv[]) {
        int a, b, i;
        char input[256], n;
        long result;

        if (argc < 3) {
                printf("usage!\n");
                return 1;
        }

        a = atoi(argv[1]);
        b = atoi(argv[2]);
        scanf("%s",&input);

        printf("%d %d\n", a, b);
        i=0;
        while(input[i]!=0) {
                /* shift cypher with formula */
                /* error check */

                if (input[i] < 0x60 || input[i] > 0x7a) {
                        printf("value not in set (a-z)!\n");
                        return 1;
                }
                result = input[i] - 0x61; /*first convert to set */

//              printf("debug: a, b, result: %d %d %li\n", a, b, result);                
                if (argc == 4 && argv[3][0] == 'e') {
                        result = ((result * a) + b) % 26;
                } else {
                        int j;
                        long p1, p2 = 0;
                        p1 = result - b;
                        if (p1 < 0)
                                p1 = abs(p1) * 25; // 25 == -1 in mod 26

                        for (j = 0; j < 26;j++) {
                                if ((j * a) % 26 == 1) {
                                        p2 = j; // p2 is mul inv of a
                                        break;
                                }
                        }
                        if (p2 == 0) {
                                printf(" no gcd for %d, %d\n", a, b);
                                return 1;
                        }
                        result = (p1 * p2) % 26;
                }

                result += 0x61;

                printf("%c",result);
                i++;
        }
        printf("\n");
}