The BEP itself is almost trivial. The difficult work is implementing it in a client that makes it useful for users and content providers.

In the wild west of the internet "update" really only means "add" because you don't want the source you barely trust to provide some data to issue an update that deletes all the previous download from that source. But you also want to avoid wasting storage so some size caps and rehashing old data to see if it's an incremental update will also be needed.