求纯c语言编写的加密文本文件的代码（异或加密也可）,该如何处理

#include <stdio.h>
FILE *fi,*fo;
void main() {
    fo=fopen("500M.out","wb");
    fi=fopen("500M.in","rb");
    fseek(fi,-1L,SEEK_END);
    while (1) {
        fputc(fgetc(fi)^0xA5,fo);
        if (fseek(fi,-2,SEEK_CUR)) break;
    }
    fclose(fi);
    fclose(fo);
}