#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define MASK_BIT_7      0x80
#define MASK_BIT_6      0x40
#define MASK_BIT_5      0x20
#define MASK_BIT_4      0x10
#define MASK_BIT_3      0x08
#define MASK_BIT_2      0x04
#define MASK_BIT_1      0x02
#define MASK_BIT_0      0x01


unsigned char next(unsigned char lfsr)
{
	lfsr = (lfsr << 1);
	if(__builtin_parity(lfsr)==0){
		lfsr ^= 1 << 0;
	}
	return lfsr;

}

int main(int argc, char *argv[])
{
	int fd_in,fd_out;
	unsigned char w,buf;;


	assert(argc >= 4);
	fd_in = open(argv[1],O_RDONLY);
	fd_out = open(argv[2],O_WRONLY|O_TRUNC|O_CREAT,0600);
	w = (unsigned char)strtol(argv[3],NULL,0);

	while(1){
		ssize_t nb = read(fd_in,&buf,1);
		if (nb <=0)
			break;
		buf ^= w;
		write(fd_out,&buf,1);
		w=next(w);
	}
	return 0;
}