104 lines
3.3 KiB
Markdown
104 lines
3.3 KiB
Markdown
<!--
|
|
#
|
|
# Licensed to the Apache Software Foundation (ASF) under one
|
|
# or more contributor license agreements. See the NOTICE file
|
|
# distributed with this work for additional information
|
|
# regarding copyright ownership. The ASF licenses this file
|
|
# to you under the Apache License, Version 2.0 (the
|
|
# "License"); you may not use this file except in compliance
|
|
# with the License. You may obtain a copy of the License at
|
|
#
|
|
# http://www.apache.org/licenses/LICENSE-2.0
|
|
#
|
|
# Unless required by applicable law or agreed to in writing,
|
|
# software distributed under the License is distributed on an
|
|
# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
|
|
# KIND, either express or implied. See the License for the
|
|
# specific language governing permissions and limitations
|
|
# under the License.
|
|
#
|
|
-->
|
|
|
|
## Image signing
|
|
|
|
This signs the image by computing hash over the image, and then
|
|
signing that hash. Signature is computed by newt tool when it's
|
|
creating the image. This signature is placed in the image trailer.
|
|
|
|
The public key of this keypair must be included in the bootloader,
|
|
as it verifies it before allowing the image to run.
|
|
|
|
This facility allows you to use multiple signing keys. This would
|
|
be useful when you want to prevent production units from booting
|
|
development images, but want development units to be able to boot
|
|
both production images and development images.
|
|
|
|
## Creating signing keys
|
|
First you need a keypair to use for signing. You can create
|
|
one with openssl command line tool.
|
|
|
|
openssl genrsa -out image_sign.pem 2048
|
|
|
|
This created a file which contains both the private and public key,
|
|
and will be used when signing images.
|
|
|
|
Then you need to extract the public key from this to include it
|
|
in the bootloader. Bootloader need to keep key parsing minimal,
|
|
so it expects simple key format.
|
|
|
|
openssl rsa -in image_sign.pem -pubout -out image_sign_pub.der -outform DER -RSAPublicKey_out
|
|
|
|
Now the public key is in file called image_sign_pub.der.
|
|
|
|
For ECDSA224 these commands are similar.
|
|
|
|
openssl ecparam -name secp224r1 -genkey -noout -out image_sign.pem
|
|
openssl ec -in image_sign.pem -pubout -outform DER -out image_sign_pub.der
|
|
|
|
And then the ECDSA256.
|
|
openssl ecparam -name prime256v1 -genkey -noout -out image_sign.pem
|
|
openssl ec -in image_sign.pem -pubout -outform DER -out image_sign_pub.der
|
|
|
|
## Creating a key package
|
|
|
|
xxd -i image_sign_pub.der image_sign_pub.c.import
|
|
|
|
Then you need to create a package containing this key, or keys.
|
|
|
|
## Sample pkg.yml
|
|
This gets bootutil to turn on image signature validation.
|
|
|
|
pkg.name: libs/mykeys
|
|
pkg.deps:
|
|
- "@apache-mynewt-core/boot/bootutil"
|
|
|
|
## Sample source file
|
|
This exports the keys.
|
|
|
|
#include <bootutil/sign_key.h>
|
|
|
|
#include "image_sign_pub.c.import"
|
|
|
|
const struct bootutil_key bootutil_keys[] = {
|
|
[0] = {
|
|
.key = image_sign_pub_der,
|
|
.len = &image_sign_pub_der_len,
|
|
}
|
|
};
|
|
|
|
const int bootutil_key_cnt = sizeof(bootutil_keys) / sizeof(bootutil_keys[0]);
|
|
|
|
## Building bootloader
|
|
|
|
Enable the MCUBOOT_SIGN_RSA syscfg setting in your app or target syscfg.yml
|
|
file
|
|
|
|
syscfg.vals:
|
|
MCUBOOT_SIGN_RSA: 1
|
|
|
|
After you've created the key package, you must include it in the build
|
|
for bootloader. So modify the pkg.yml for apps/boot to include it.
|
|
|
|
The syscfg variable to enable ECDSA224 is MCUBOOT_SIGN_EC, and
|
|
MCUBOOT_SIGN_EC256 for ECDS256.
|