#include <stddef.h>

// In-place ROT13 cipher
static void 
rot13(unsigned char *b, size_t n)
{
    for (size_t i = 0; i < n; i++) {
        switch (b[i]) {
        case 'a'...'z': 
            b[i] += 13;
            if (b[i] > 'z') b[i] -= 'z' - 'a' + 1;
            break;

        case 'A'...'Z': 
            b[i] += 13;
            if (b[i] > 'Z') b[i] -= 'Z' - 'A' + 1;
            break;
        }
    }
}