Chunk has:

  -- raw digest (always)

  -- file    (many)
  -- offset  (many)
  -- length  (many)

  -- enc digest (many)

Chunk description in .bracup 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

----

PositionedChunk  (subclass of RawChunk)
 - has a:
     file
     offset
     RawChunk
 - used by:
     $file->foreach_chunk(sub { my $poschunk = shift; });
     restoring stuff?
 - can:
     write back to disk?

RawChunk
 - has a:
     length
     digest
     contents
 - used by:
     positionedchunk.

ChunkHandle
 - has a:
     digest of stored chunk
 - used by:
     return value from asking target if it has a raw chunk,
     or after it stores a raw chunk.

StoredChunk






------

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.

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)

---

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.  :-)