A git-annex key has this format:

	BACKEND[-sNNNN][-mNNNN][-SNNNN-CNNNN]--NAME

For example:

	SHA256E-s31390--f50d7ac4c6b9031379986bc362fcefb65f1e52621ce1708d537e740fefc59cc0.mp3

* The backend is one of the [[key-value_backends|backends]], which
  are always upper-cased.
* The name field at the end has a format dependent on the backend. It is
  always the last field, and is prefixed with "--". Unlike other fields,
  it may contain "-" in its content. It should not contain newline
  characters or "/"; otherwise nearly anything goes. 
* The "-s" field is optional, and is the size of the content in bytes.
* The "-m" field is optional, and is the mtime of the file when it was
  added to git-annex, expressed as seconds from the epoch.
  This is currently only used by the WORM backend.
* The "-S" and "-C" fields are only used for keys that are chunks
  of some other key. "-S" is the size of the chunk, and "-C" is the chunk
  number (starting at 1).
* Other fields could be added in the future, if needed.

git-annex always puts the fields in the order shown above when serializing
a key. It can parse keys with the fields in other orders (although the name
field must always come last).

The `git annex examinekey` command can be used to extract information from
a key.