mirror of https://github.com/dswd/zvault
80 lines
3.4 KiB
Markdown
80 lines
3.4 KiB
Markdown
% Bundle file format
|
|
## Bundle file format
|
|
|
|
The bundle file format consists of 4 parts:
|
|
- A magic header with version
|
|
- An encoded header structure
|
|
- An encoded chunk list
|
|
- The chunk data
|
|
|
|
The main reason for having those multiple parts is that it is expected that the
|
|
smaller front parts can be read much faster than the the whole file. So
|
|
information that is needed more frequently is put into earlier parts and the
|
|
data that is need the least frequent is put into the latter part so that it does
|
|
not slow down reading the front parts. Keeping those parts in separate files
|
|
was also considered but rejected to increase the reliability of the storage.
|
|
|
|
|
|
### Magic header with version
|
|
The first part of a bundle file contains an 8 byte magic header with version
|
|
information.
|
|
|
|
The first 6 bytes of the header consist of the fixed string "zvault", followed
|
|
by one byte with the fixed value 0x01. Those 7 bytes make up the magic header of
|
|
the file and serve to identify the file type as a zvault bundle file.
|
|
|
|
The 8th byte of the first file part is the version of the file format. This
|
|
value is currently 0x01 and is expected to be increased for any breaking changes
|
|
in the file format.
|
|
|
|
|
|
### Encoded header structure
|
|
The encoded header structure is the second part of the bundle file format and
|
|
follows directly after the 8 bytes of the magic header.
|
|
|
|
The header structure is defined in `bundle.rs` as `BundleInfo` and contains
|
|
general information on the bundle's contents and on how to decode the other two
|
|
parts of the bundle file.
|
|
|
|
This header structure is encoded using the *MsgPack* format. It is neither
|
|
compressed (since its size is pretty small) nor encrypted (since it only
|
|
contains general information and no user data) in any way.
|
|
|
|
|
|
### Encoded chunk list
|
|
The chunk list is the third part of the bundle file and follows directly after
|
|
the encoded header structure.
|
|
|
|
The chunk list contains hashes and sizes of all chunks stored in this bundle in
|
|
the order they are stored. The list is encoded efficiently as 20 bytes per chunk
|
|
(16 for the hash and 4 for the size) as defined in `../util/chunk.rs`.
|
|
|
|
Since the chunk list contains confidential information (the chunk hashes and
|
|
sized can be used to identify files) the encoded chunk list is encrypted using
|
|
the encryption method specified in the header structure. The header structure
|
|
also contains the full size of the encoded and encrypted chunk list which is
|
|
needed since the encryption could add some bytes for a nonce or an
|
|
authentication code.
|
|
|
|
The chunk list is not compressed since the hashes have a very high entropy and
|
|
do not compress significantly.
|
|
|
|
The chunk list is not stored in the header structure because it contains
|
|
confidential data and the encryption method is stored in the header. Also the
|
|
chunk list can be pretty big compared to the header which needs to be read more
|
|
often.
|
|
|
|
|
|
### Chunk data
|
|
The chunk data is the final part of a bundle file and follows after the encoded
|
|
chunk list. The starting position can be obtained from the header as the encoded
|
|
size of the chunk list is stored there.
|
|
|
|
The chunk data part consists of the content data of the chunks contained in this
|
|
bundle simply concatenated without any separator. The actual size (and by
|
|
summing up the sizes also the starting position) of each chunk can be obtained
|
|
from the chunk list.
|
|
|
|
The chunk data is compressed as whole (solid archive) and encrypted with the
|
|
methods specified in the bundle header structure.
|