Great start! You could also try something like some sort of simple password key.
Just to maybe give you a nudge in the direction you are wishing to go.
#define BUF_SIZE 256
std::fgets(buffer, BUF_SIZE, stdin);
std::FILE *in, *out;
in = std::fopen("gonder", "rb");
out = std::fopen("yeni", "wb");
if(in && out)
int size = std::strlen(buffer), i = 0, ch;
if((ch = std::fgetc(in)) != EOF)
std::putc(ch ^ buffer[i % size], out);