firmware-utils: add nec-enc

nec-enc provides firmware encoding/decoding with model specific key
for NEC devices.

known devices:

  - Aterm WF1200CR
  - Aterm WG1200CR
  - Aterm WG2600HS

usage:

  nec-enc -i infile -o outfile -k key

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
[checkpatch fixes, marked usage as noreturn, added static function,
moved buf* from stack to the global data segment]

tools/firmware-utils/Makefile

@@ -85,6 +85,7 @@ define Host/Compile
 	$(call cc,mkdlinkfw mkdlinkfw-lib, -lz -Wall --std=c99)
 	$(call cc,dns313-header, -Wall)
 	$(call cc,mksercommfw, -Wall)
+	$(call cc,nec-enc, -Wall)
 endef
 
 define Host/Install

tools/firmware-utils/src/nec-enc.c

@@ -0,0 +1,129 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * nec-enc.c - encode/decode nec firmware with key
+ *
+ * based on xorimage.c in OpenWrt
+ *
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <unistd.h>
+
+#define KEY_LEN     16
+#define PATTERN_LEN 251
+
+static int
+xor_pattern(uint8_t *data, size_t len, const char *key, int k_len, int k_off)
+{
+	int offset = k_off;
+
+	while (len--) {
+		*data ^= key[offset];
+		data++;
+		offset = (offset + 1) % k_len;
+	}
+
+	return offset;
+}
+
+static void xor_data(uint8_t *data, size_t len, const uint8_t *pattern)
+{
+	for (int i = 0; i < len; i++) {
+		*data ^= pattern[i];
+		data++;
+	}
+}
+
+static void __attribute__((noreturn)) usage(void)
+{
+	fprintf(stderr, "Usage: nec-enc -i infile -o outfile -k <key>\n");
+	exit(EXIT_FAILURE);
+}
+
+static unsigned char buf_pattern[4096], buf[4096];
+
+int main(int argc, char **argv)
+{
+	int k_off = 0, ptn = 0, c, ret = EXIT_SUCCESS;
+	char *ifn = NULL, *ofn = NULL, *key = NULL;
+	size_t n, k_len;
+	FILE *out, *in;
+
+	while ((c = getopt(argc, argv, "i:o:k:h")) != -1) {
+		switch (c) {
+		case 'i':
+			ifn = optarg;
+			break;
+		case 'o':
+			ofn = optarg;
+			break;
+		case 'k':
+			key = optarg;
+			break;
+		case 'h':
+		default:
+			usage();
+		}
+	}
+
+	if (optind != argc || optind == 1) {
+		fprintf(stderr, "illegal arg \"%s\"\n", argv[optind]);
+		usage();
+	}
+
+	in = fopen(ifn, "r");
+	if (!in) {
+		perror("can not open input file");
+		usage();
+	}
+
+	out = fopen(ofn, "w");
+	if (!out) {
+		perror("can not open output file");
+		usage();
+	}
+
+	if (!key) {
+		fprintf(stderr, "key is not specified\n");
+		usage();
+	}
+
+	k_len = strnlen(key, KEY_LEN + 1);
+	if (k_len == 0 || k_len > KEY_LEN) {
+		fprintf(stderr, "key length is not in range (0,%d)\n", KEY_LEN);
+		usage();
+	}
+
+	while ((n = fread(buf, 1, sizeof(buf), in)) > 0) {
+		for (int i = 0; i < n; i++) {
+			buf_pattern[i] = ptn + 1;
+			ptn++;
+
+			if (ptn > 250)
+				ptn = 0;
+		}
+
+		k_off = xor_pattern(buf_pattern, n, key, k_len, k_off);
+		xor_data(buf, n, buf_pattern);
+
+		if (fwrite(buf, 1, n, out) != n) {
+			perror("failed to write");
+			ret = EXIT_FAILURE;
+			goto out;
+		}
+	}
+
+	if (ferror(in)) {
+		perror("failed to read");
+		ret = EXIT_FAILURE;
+		goto out;
+	}
+
+out:
+	fclose(in);
+	fclose(out);
+	return ret;
+}