[rfk]

Any sort of verification presumes that you have an initial trusted checksum against which to verify, so I don't believe this solves the separate problem of obtaining such a checksum.

IIUC your question here boils down to "what is the point of a hash tree?" as opposed to e.g. a list of individual chunk hashes. The answer is that a hash tree lets you verify an individual chunk by looking at the hashes of O(log(N)) chunks rather than having to look at the hash of every chunk.

[ebg13]

Thanks. That is meaningful, but I still don't understand how "the recipient can stream a video attachment, while still verifying each byte as it comes in" isn't basically also true for sequential hashing with periodic chunk hashes.