firmware: add tool for signing d-link ru router factory firmware images

Some Russian d-link routers require that their firmware be signed with a
salted md5 checksum followed by the bytes 0x00 0xc0 0xff 0xee. This tool
signs factory images the OEM's firmware accepts them.

Signed-off-by: Andrew Pikler <andrew.pikler@gmail.com>

target/linux/ramips/image/Makefile

@@ -148,6 +148,11 @@ define Build/sercom-seal
 		$(1)
 endef
 
+define Build/sign-dlink-ru
+	sign_dlink_ru $@ $1 $2
+	mv $@.new $@
+endef
+
 define Build/trx
 	$(STAGING_DIR_HOST)/bin/trx $(1) \
 		-o $@ \

tools/firmware-utils/Makefile

@@ -84,6 +84,7 @@ define Host/Compile
 	$(call cc,pc1crypt)
 	$(call cc,ptgen cyg_crc32)
 	$(call cc,seama md5)
+	$(call cc,sign_dlink_ru md5,-Wall)
 	$(call cc,spw303v)
 	$(call cc,srec2bin)
 	$(call cc,tplink-safeloader md5,-Wall --std=gnu99)

tools/firmware-utils/src/sign_dlink_ru.c

@@ -0,0 +1,225 @@
+/*
+ * This program is designed to sign firmware images so they are accepted
+ * by D-Link DIR-882 R1 WebUIs.
+ *
+ * Copyright (C) 2020 Andrew Pikler
+ * 
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ * 
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this program; If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+
+#include "md5.h"
+
+#define BUF_SIZE 4096
+#define MD5_HASH_LEN 16
+
+
+typedef struct _md5_digest_t {
+	uint8_t digest[MD5_HASH_LEN];
+} md5_digest_t;
+
+typedef struct _salt_t {
+	char* salt_ascii;
+	uint8_t* salt_bin;
+	size_t salt_bin_len;
+} salt_t;
+
+void read_file_bytes(FILE* f, MD5_CTX* md5_ctx) {
+	uint8_t buf[BUF_SIZE];
+	size_t bytes_read;
+	rewind(f);
+	
+	while (0 != (bytes_read = fread(buf, sizeof(uint8_t), BUF_SIZE, f))) {
+		MD5_Update(md5_ctx, buf, bytes_read);
+	}
+
+	if (!feof(f)) {
+		printf("Error: expected to be at EOF\n");
+		exit(-1);
+	}
+}
+
+void add_magic_bytes(FILE* f) {
+	char magic_bytes[] = { 0x00, 0xc0, 0xff, 0xee };
+	size_t magic_bytes_len = 4;
+	fwrite(magic_bytes, magic_bytes_len, 1, f);
+}
+
+/**
+ * Add the signature produced by this salt to the file
+ * The signature consists by creating an MD5 digest wht the salt bytes plus
+ * all of the bytes in the firmware file, then adding the magic bytes to the
+ * file
+ */
+void add_signature(FILE* f, salt_t* salt) {
+	md5_digest_t digest;
+	MD5_CTX md5_context;
+
+	MD5_Init(&md5_context);
+	MD5_Update(&md5_context, salt->salt_bin, salt->salt_bin_len);
+	read_file_bytes(f, &md5_context);
+	MD5_Final(digest.digest, &md5_context);
+
+	fwrite(&digest.digest, sizeof(uint8_t), MD5_HASH_LEN, f);
+	add_magic_bytes(f);
+}
+
+void add_version_suffix(FILE* f) {
+	char* version_suffix = "c0ffeef0rge";
+	fseek(f, 0, SEEK_END);
+	fwrite(version_suffix, sizeof(char), strlen(version_suffix), f);
+}
+
+int asciihex_to_int(char c) {
+	if(c >= '0' && c <= 'F')
+		return c - '0';
+
+	if(c >= 'a' && c <= 'f')
+		return 10 + c - 'a';
+	return -1;
+}
+
+/**
+ * Verify this is a valid hex string to convert
+ */
+void verify_valid_hex_str(char* s) {
+	int i;
+	int s_len = strlen(s);
+	if (s_len == 0) {
+		printf("invalid empty salt: %s\n", s);
+		exit(-1);
+	}
+
+	if (s_len % 2 != 0) {
+		printf("invalid odd len salt: %s\n", s);
+		exit(-1);
+	}
+
+	for (i = 0; i < s_len; ++i) {
+		if (asciihex_to_int(s[i]) < 0) {
+			printf("invalid salt (invalid hex char): %s\n", s);
+			exit(-1);
+		}
+	}
+}
+
+/**
+ * Convert a hex ascii string to an allocated binary array. This array must be free'd
+ */
+uint8_t* convert_hex_to_bin(char * s) {
+	int i;
+	int s_len = strlen(s);
+
+	uint8_t* ret = malloc(s_len / 2);
+	for (i = 0; i < s_len; i += 2) {
+		ret[i / 2] = (asciihex_to_int(s[i]) << 4) | asciihex_to_int(s[i + 1]);
+	}
+
+	return ret;
+}
+
+void init_salt(salt_t* salt, char * salt_ascii) {
+	salt->salt_ascii = salt_ascii;
+	salt->salt_bin = convert_hex_to_bin(salt_ascii);
+	salt->salt_bin_len = strlen(salt_ascii) / 2;
+}
+
+void free_salt(salt_t* salt) {
+	free(salt->salt_bin);
+}
+
+/**
+ * Verify that the arguments are valid, or exit with failure
+ */
+void verify_args(int argc, char** argv) {
+	int i;
+
+	if (argc < 3) {
+		printf("Usage: %s <firmware file> <signing hash1> <signing hash2> ... <signing hash n>\n", argv[0]);
+		exit(1);
+	}
+
+	for (i = 2; i < argc; i++) {
+		verify_valid_hex_str(argv[i]);
+	}
+}
+
+FILE* make_out_file(char* filename) {
+	uint8_t buf[BUF_SIZE];
+	int bytes_read;
+	char* suffix = ".new";
+	int new_filename_len = strlen(filename) + strlen(suffix) + 1;
+	char* new_filename = malloc(new_filename_len);
+	strcpy(new_filename, filename);
+	strcat(new_filename, suffix);
+
+	FILE* f = fopen(filename, "r+");
+	if (!f) {
+		printf("cannot open file %s\n", filename);
+		exit(2);
+	}
+
+	FILE* out = fopen(new_filename, "w+");
+	free(new_filename);
+	if (!out) {
+		printf("cannot open file %s\n", filename);
+		exit(2);
+	}
+
+	while (0 != (bytes_read = fread(buf, sizeof(uint8_t), BUF_SIZE, f))) {
+		fwrite(buf, sizeof(uint8_t), bytes_read, out);
+	}
+	fclose(f);
+	return out;
+}
+
+/**
+ * Sign the firmware file after all of our checks have completed
+ */
+void sign_firmware(char* filename, char** salts, int num_salts) {
+	int i;
+	salt_t salt;
+	FILE* f = make_out_file(filename);
+
+	// add a version suffix string - dlink versions do something similar before the first signature
+	add_version_suffix(f);
+
+	//for each of the salts we are supplied with
+	for (i = 0; i < num_salts; i++) {
+		char* salt_str = salts[i];
+		// convert this str to binary
+		init_salt(&salt, salt_str);
+
+		// add the signature to the firmware file produced from this salt
+		add_signature(f, &salt);
+		free_salt(&salt);
+		printf("Signed with salt: %s\n", salt_str);
+	}
+
+	fclose(f);
+}
+
+
+int main(int argc, char ** argv) {
+	verify_args(argc, argv);
+	sign_firmware(argv[1], argv+2, argc-2);
+	return 0;
+}