secure boot on ASUS MeMO Pad 8/10

we don't have enough information about Rockchip secure boot. we also don't know how ASUS use it on their pads.

what we know is,

• bootloader checks signature in boot.img or recovery.img with RSA public key
• DRM KEY BOX is enabled only if signature is generated with proper RSA private key
• boot.img or recovery.img without signature can be booted, but DRM KEY BOX is disabled
• ASUS official boot.img and recovery.img are signed

what we don't know is,

• which apps/services use DRM KEY BOX
• ... and everything else