/* Xors the input with a hex key.
 * Author: Peter Wu <lekensteyn@gmail.com>
 * Date: 2013-10-05
 */
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>

static inline int hex_to_char(unsigned char c1, unsigned char c2, unsigned char *dst) {
	const char str[3] = { c1, c2, '\0' };
	char *end;

	*dst = strtoul(str, &end, 16);
	if (*end != '\0') {
		fprintf(stderr, "Not a hex octet %s\n", str);
		return 1;
	}

	return 0;
}

size_t parse_hex(const char *str, unsigned char **key) {
	size_t len = strlen(str);
	size_t i;

	if (len == 0) {
		fputs("The key must be non-empty\n", stderr);
		return 0;
	}
	if (len % 2 != 0) {
		fputs("Key length must be a multiple of 2\n", stderr);
		return 0;
	}

	*key = malloc(len / 2);
	if (!*key) {
		abort();
	}

	for (i = 0; i < len / 2; i += 2) {
		if (hex_to_char(str[i], str[i + 1], &(*key)[i / 2])) {
			goto fail;
		}
	}

	return len / 2;
fail:
	free(*key);
	return 0;
}

int main(int argc, char **argv) {
	unsigned char *key;
	int c;
	size_t key_len, key_i;
	if (argc < 2) {
		fputs("Usage: xor hexkey\n", stderr);
		return 1;
	}

	key_len = parse_hex(argv[1], &key);
	if (key_len == 0) {
		return 1;
	}

	while ((c = getchar()) != EOF) {
		putchar(((unsigned char) c) ^ key[key_i]);
		key_i = (key_i + 1) % key_len;
	}

	free(key);

	return 0;
}