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## 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 BOOTUTIL_SIGN_RSA syscfg setting in your app or target syscfg.yml
file

    syscfg.vals:
        BOOTUTIL_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 BOOTUTIL_SIGN_EC, and
BOOTUTIL_SIGN_EC256 for ECDS256.