Chunk description in .brackup file:
  Chunks:  offset;raw_length;stored_length;typed_digest
Proposal to change:
  Chunks:  offset;raw_length;stored_length;typed_digest(stored);typed_digest(raw);flags

Where flags is comma separate list of \w+.  e.g.  "gz" for gzip compression

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Document:  purpose of chunks named by their final digest is twofold:
   1) can verify integrity of storage medium.  is it corrupt?
   2) hides proof of ownership of contents (when encrypted)
side-effect:
   -- when we do compression, we'll be consistent and store it as its
      compressed digest, even if not encrypted as well.

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Maybe we don't need per-chunk meta files:
   -- can get it all from .brackup (meta)files on the server.
      (TODO: abstract out parser for multiple users)

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smart chunk-sizing on certain files w/ metadata and data separate:
like mp3 files and their id3.  have a smart chunker that's the data
part vs. the id3 part, so updating id3 later doesn't reupload the
entire data part.  :-)