#include <stdio.h>
#include <stdlib.h>

int
main(int argc, char *argv[])
{
	long offset;
	int i;
	unsigned char c1, c2;
	FILE *fp;

	if (argc != 3) {
		fprintf(stderr, "Usage: otp keyfile offset\n");
		exit(1);
	} 

	if (!(fp = fopen(argv[1], "r"))) {
		perror(argv[1]);
		exit(1);
	}
	offset = strtol(argv[2], NULL, 10);
	if (fseek(fp, offset, SEEK_SET) == -1) {
		fprintf(stderr, "key too short\n");
		exit(2);
	}
		
	while ((i = getchar()) != EOF) {
		c1 = (unsigned char) i;
		if ((i = fgetc(fp)) == EOF) {
			fprintf(stderr, "key too short\n");
			exit(2);
		}
		c2 = (unsigned char) i;
		putchar(c1 ^ c2);
	}
	fprintf(stderr, "%ld\n", ftell(fp));
	return 0;
}