xor: xor the input with a hex key

1 files changed, 75 insertions, 0 deletions

diff --git a/xor.c b/xor.c
new file mode 100644
index 0000000..934ba4d
--- /dev/null
+++ b/xor.c
@@ -0,0 +1,75 @@
+/* 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;
+}