A git-annex key has this format:

BACKEND-sNNNN-mNNNN--NAME

For example:

SHA256E-s31390--f50d7ac4c6b9031379986bc362fcefb65f1e52621ce1708d537e740fefc59cc0.mp3

• The backend is one of the key-value 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; 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.
• Other fields could be added in the future, if needed.
• Fields may appear, in any order (though always before the name field).