Simple ROT13 Cipher

rot13.c

#include <stdio.h>

#define IS_ALPHA(c) ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z'))

char *rot13_cipher(char *dst, char *src)
{
    char *tmp = dst;

    while (*src) {

        if (!IS_ALPHA(*src))
            *tmp++ = *src++;
        
        if (*src >= 'a' && *src < 'n' || *src >= 'A' && *src < 'N')
            *tmp++ = *src++ + 13;
        
        if (*src >= 'n' && *src <= 'z' || *src >= 'N' && *src <= 'Z')
            *tmp++ = *src++ - 13;
    }

    *tmp = 0;

    return (dst);
}

int main(void)
{
   char buf[0x100];
  
   rot13_cipher(buf, "h3llo/ W0rld");
  
   printf("%s\n", buf);
   
   return (0);
}