The way Android goes about encrypting data on SD cards is vague to me. I've read about ASEC files, OBB files, /data/misc/systemkeys/AppsOnSD.sks, and keystores. I know that keys used to encrypt SD card data are stored on a device's internal memory module and ASEC fies are dm-crypt volumes, but it is unclear how all of this is related.
Are ASEC files used to encrypt applications installed on SD cards or data stored by applications on SD cards? If the former, then what is the point of that? If it is DRM, then how are the DRM keys delivered to the device? How do users choose to encrypt their SD cards and what mechanism is used to accomplish that? Are OBB files similar to ASEC files?
