OAD Storage & Security

There are a couple of aspects with OAD that one must take into account when choosing which type of OAD implementation suits your application.

One aspect is where a new downloaded image is to be stored during the OAD process, for which you have three methods: on-chip, off-chip, dual-image. On-chip OAD and dual-image OAD both store the downloaded image to internal flash, while off-chip OAD stores the downloaded image to external flash.

Another aspect is security, which is to whether or not authenticate a new downloaded image to be from a trusted source. This OAD solution uses MCUBoot, hence, it is not possible to do OAD without security. Secure OAD authenticates new images using an Elliptic Curve Digital Signature Algorithm (ECDSA) before it is executed on-chip.

OAD Storage

During an OAD procedure the candidate image is stored either in the internal flash when using On-chip OAD or Dual-image OAD, or in external flash when using off-chip OAD. On-chip, dual-image and off-chip OAD have associated trade-offs and benefits, which are summarized in the “OAD storage comparison” table below.

The biggest advantage of using on-chip OAD and dual-image OAD is allowing a single-chip OAD solution. However, on-chip OAD and dual-image OAD both put restrictions on application size in terms of flash usage.

Off-chip OAD does not have these restrictions, and can store multiple application images and a factory image in the external flash without limiting the application size on the internal flash. This, of course, comes at the cost of requiring an additional component.

Despite their differences, the different OAD types are invariant to the transport of candidate images.

Table 32. OAD method comparison

Method	Advantages	Limitations
On-chip	No external flash required Download is performed by persistent application leaving more flash for incoming image.	Application size restriction Persistent application is required.
Off-chip	Ability to store multiple / backup images App + Stack merged update supported Maximum application size available to customer	Low power external flash required
Dual-Image	No external flash required	Application size restriction

OAD Security

When talking about security in the context of OAD, it is referred to whether or not new downloaded images must be authenticated as part of the OAD process. It does not refer to the security level of the actual transport mechanism of the OAD image. The main purpose for authenticating new downloaded images is to verify the image is from an entrusted source. Secure OAD will always authenticate new images as part of the OAD process when properly implemented along with the secure bootloader.

Secure OAD uses Elliptic Curve Digital Signature Algorithm (ECDSA) to sign and verify secure OAD images. More specifically, the ECDSA-P256 algorithm is implemented. The underlying authentication mechanisms for ECDSA is that a suitable private and public key pair are generated, and that the private key used for image signing is thoroughly protected. Only the public key is stored on Target device.

Warning

It is imperative that the system developer protect the corresponding private key. Since any party that has access to the private key can generate signed images, these images will be treated as a valid authentic image and executed by the device. Likewise, if the private key is lost, the ability to generate new valid firmware images is lost.