ROT13 (https://en.wikipedia.org/wiki/ROT13)

rot13.c

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

char rot13_letter(char letter);

int main(int argc, char **argv)
{
    int i;
    char *c;

    for (i = 1; i < argc; ++i) {
        c = argv[i];
        while (*c)
            fputc(rot13_letter(*(c++)), stdout);
        fputc(i == argc - 1 ? '\n' : ' ', stdout);
    }

    exit(EXIT_SUCCESS);
}

char rot13_letter(char letter)
{
    short int cmp_letter = 13;
    short int rot13_offset;
    char rot13 = letter;

    if (!isalpha(letter))
        return letter;

    cmp_letter += isupper(letter) ? 'A' : 'a';

    rot13_offset = letter < cmp_letter ? 1 : -1;
    rot13 += (char) (13 * (int) rot13_offset);

    if (!isalpha(rot13) || isupper(letter) != isupper(rot13))
        return '?';

    return rot13;
}