affine cipher

/**
Affine cipher (WTFPL 2014 Jake Sebastian-Jones)
*/
#include<stdio.h>
#include<stdlib.h>

char inverse_values[26] = {0, 1, 0, 9, 0, 21, 0, 15, 0, 3, 0, 19, 0,
                           0, 0, 7, 0, 23, 0, 11, 0, 5, 0, 17, 0, 25};

char encrypt(char x, int a, int b) {
    if( x < 'A' || (x > 'Z' && x < 'a') || x > 'z' ) return x;
    int oset = x <= 'Z' ? 'A' : 'a';
    return (a * (x - oset) + b) % 26 + oset;
}

char decrypt(char x, int inv, int b) {
    if( x < 'A' || (x > 'Z' && x < 'a') || x > 'z' ) return x;
    int oset = (x <= 'Z' ? 'A' : 'a');
    int chk = inv * ((x - oset) - b) % 26;
    return (chk >= 0 ? chk : 26 + chk) + oset;
}

int main(int argc, char **argv) {
    char c, inv;
    int a = atoi(argv[1]);
    int b = atoi(argv[2]) % 26;

    if(a > 25 || (inv = inverse_values[a]) == 0) {
        fprintf(stderr, "Error: 'a' value must be less than "
                        "and coprime with 26\n");
        return 64;
    }

    while( (c = fgetc(stdin)) != EOF) {
        #ifdef DECRYPT
            fprintf(stdout, "%c", decrypt(c, inv, b));
        #else
            fprintf(stdout, "%c", encrypt(c, a, b));
        #endif
    }
}