Asymmetrically Encrypted Update Images

Asymmetrically encrypted update images are realized by an asymmetrical encrypted sw-description, making it possible to decrypt images device specific. The artifacts themselves are still encrypted symmetrically. An AES key must be provided in the sw-description. At the moment only Cryptographic Message Syntax (CMS) is available for asymmetic decryption.

Use Cases

Asymmetrically encrypted update images, with individual device key pairs, are inherently more secure than a purely symmetrical solution, because one compromised private device key does not affect the security of the others.
If CONFIG_SIGNED_IMAGES is enabled too and a device’s private key is compromised, the key pair can be excluded from the list of eligible devices for receiving new update images.
The AES key can be securely exchanged with each new update image, as it is part of the sw-description, even in the absence of direct access to the device.

Create a Self-Signed Device Key Pair

As an example, an elliptic curve key pair (PEM) is generated for a single device. These steps must be repeated for all other devices. An RSA key pair could be used in the same way.

# Create a private key and a self-signed certificate
openssl ecparam -name secp521r1 -genkey -noout -out device-key-001.pem
openssl req -new -x509 -key device-key-001.pem -out device-cert-001.pem -subj "/O=SWUpdate /CN=target"

# Combine the private key and the certificate into a single file
# Note: The certificate is optional, but it speeds up key matching,
# especially when there are many different device keys.
cat device-key-001.pem device-cert-001.pem > device-001.pem

Symmetric Encryption of Artifacts

Generate an AES key and IV, as familiar from symmetric image encryption. The encryption process for the artifacts remains unchanged.

Encryption of sw-description for Multiple Devices

All device certificates togther are used for encryption.

# Encrypt sw-description for multiple devices
openssl cms -encrypt -aes-256-cbc -in sw-description.in -out sw-description -outform DER $(printf ' -recip %q' certs/device-cert-*.pem)

Assume sw-description.in is the plaintext input and sw-description is the encrypted output. After -recip, a list of all device certificates is expected. One for each device you want to encrypt for.

Decryption of sw-description for a Single Device

The combined key pair (private key and certificate) is used for decryption. SWUpdate handles the decryption process autonomously. Manually executing this step is not necessary and is provided here solely for development purposes.

# Decrypt sw-description for a single device
openssl cms -decrypt -in sw-description  -out sw-description.out -inform DER -inkey device-privatekey-001.pem -recip device-cert-001.pem

Assume sw-description is the encrypted sw-description and sw-description.out is the plaintext output. The device private key is required for decryption, while the certificate itself is optional, but helps speed up the key matching process.

Example Asymmetrically Encrypted Image

The image artifacts should be symmetrically encrypted and signed in advance. Now, create a plain sw-description.in file. The attributes: encrypted, aes-key and ivt are necessary for artifact decryption. It is okay that the AES key is included here, because the sw-description file will be encrypted afterwards.

software =
{
    version = "1.0.0";
    images: ({
        filename = "rootfs.ext4.enc";
        device = "/dev/mmcblk0p3";
        sha256 = "131159df3a4efaa890ff80173664a125c496c458dd432a8a6acae18872e35822";
        encrypted = "aes-cbc";  // former: encrypted = true;
        aes-key = "ed73b9d3bf9c655d5a0b04836d8be48660a4a4bb6f4aa07c6778e00e342881ac";
        ivt = "ea34a55a0c3476ed78f238ac87a7970c";
    });
}

Sign sw-description.in and create the signature file sw-description.sig:

openssl cms -sign -in sw-description.in -out sw-description.sig -signer signing-cert.pem -inkey signing-key.pem -outform DER -nosmimecap -binary

Asymmetrically encrypt the sw-description for multiple devices:

openssl cms -encrypt -aes-256-cbc -in sw-description.in -out sw-description -outform DER $(printf ' -recip %q' certs/device-cert-*.pem)

Create the new update image (SWU):

#!/bin/sh

FILES="sw-description sw-description.sig rootfs.ext4.enc"

for i in $FILES; do
    echo $i;done | cpio -ov -H crc >  firmware.swu

Running SWUpdate with Asymmetrically Encrypted Images

Asymmetric encryption support can be enabled by configuring the compile-time option CONFIG_ASYM_ENCRYPTED_SW_DESCRIPTION, which depends on CONFIG_ENCRYPTED_SW_DESCRIPTION. To pass the combined key pair (PEM) generated earlier to SWUpdate, use the -K argument. Alternatively, use the decryption-key parameter in the swupdate.cfg.

Security Considerations

Ideally, generate the private key on the device during factory provisioning, ensuring it never leaves the device. Only the public certificate leaves the device for encrypting future update packages.
This feature should be used in conjunction with signature verification (CONFIG_SIGNED_IMAGES) to ensure data integrity. In principle, anyone with the corresponding device certificate can create update packages.
As a side effect, the size of the update package may significantly increase in a large-scale deployment. To enhance scalability, consider using group keys. Smaller groups should be preferred over larger ones. For example, 1000 device keys (using secp521r1) increase the sw-description size to 0.35 MB. This means that by forming groups of 1000, it is possible to support 1 million devices. Alternatively, groups of 100 increase the sw-description size to 3.5 MB accordingly.
Exchange the AES key in the sw-description with each update package.
Avoid encrypting new update packages for compromised devices, if there is no direct access to the device or if unauthorized users have access to new update packages.