#include <stdio.h>

int main(int argc, char *argv[])
{
    FILE *fi, *fo;
    char *cp;
    int c;

    if ((cp = argv[1]) && *cp!='\0') {
        if ((fi = fopen(argv[2], "rb")) != NULL) {
            if ((fo = fopen(argv[3], "wb")) != NULL) {
                while ((c = getc(fi)) != EOF) {
                    if (!*cp) cp = argv[1];
                    c ^= *(cp++);
                    putc(c,fo);
                }
                fclose(fo);
            }
            fclose(fi);
        }
    }
}