Just rot Alphabet.

caesar.c

#include <stdio.h>
#define key 13

int main()
{
    char c;
    while(scanf("%c",&c) != EOF){
        if('A' <= c && c <= 'Z' ){
            if((c+key) > 'Z')
                c += (key%26-26);
            else
                c += key%26;
        }
        else if('a' <= c && c <= 'z'){
            if((c+key) > 'z')
                c += (key%26-26);
            else
                c += key%26;
        }
        printf("%c", c);
    }
    return 0;
}