[WIP] Draft RIOT Image Metadata

A RIOT image is appended with metadata in the format below:

typedef struct firmware_metadata {
    uint32_t magic_number;              /**< metadata magic_number (always "RIOT")  */
    uint32_t appid;                     /**< Integer representing the application ID*/
    uint32_t version;                   /**< Integer representing firmware version  */
    uint32_t size;                      /**< Size of firmware image                 */
    uint32_t start_addr;                /**< Start address in flash                 */
    uint32_t chksum;                    /**< checksum of metadata                   */
    uint8_t hash[SHA256_DIGEST_LENGTH]; /**< SHA256 Hash of firmware image          */
    uint8_t sig[FIRMWARE_SIG_LEN];      /**< Firmware signature                     */
    uint32_t pad[FIRMWARE_PADDING];     /**< padding to total of FIRMWARE_METADATA_SIZE
                                             bytes */
} firmware_metadata_t;

Where:

• uint32_t magic_number Represents a magic number to identify the metadata, it is always "RIOT".

• uint32_t appid; An application ID to identify the application type and the platform. It is "unique" for the given application.

• uint32_t version The version of the application encoded in a 32bit integer. It increases as the application is updated.

• uint32_t size Size of only the application image, without this metadata.

• uint32_t start_addr The address where the application should be installed.

• uint32_t chksum A checksum for the metadata struct (integrity check).

• uint8_t hash[SHA256_DIGEST_LENGTH] The hash values using SHA-256 algorithm over the whole firmware image.

• uint8_t sig[FIRMWARE_SIG_LEN] The signature over the hash values. It only supports Ed25519 for now.

The current apporach hashes and signs the whole image, metadata included.