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doc: Add repository compression support documentation
Co-authored-by: Michael Eischer <michael.eischer@fau.de>
This commit is contained in:
Alexander Neumann
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doc/design.rst
121
doc/design.rst
@ -62,18 +62,21 @@ like the following:
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.. code:: json
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{
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"version": 1,
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"version": 2,
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"id": "5956a3f67a6230d4a92cefb29529f10196c7d92582ec305fd71ff6d331d6271b",
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"chunker_polynomial": "25b468838dcb75"
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}
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After decryption, restic first checks that the version field contains a
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version number that it understands, otherwise it aborts. At the moment,
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the version is expected to be 1. The field ``id`` holds a unique ID
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which consists of 32 random bytes, encoded in hexadecimal. This uniquely
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identifies the repository, regardless if it is accessed via SFTP or
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locally. The field ``chunker_polynomial`` contains a parameter that is
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used for splitting large files into smaller chunks (see below).
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version number that it understands, otherwise it aborts. At the moment, the
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version is expected to be 1 or 2. The list of changes in the repository
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format is contained in the section "Changes" below.
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The field ``id`` holds a unique ID which consists of 32 random bytes, encoded
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in hexadecimal. This uniquely identifies the repository, regardless if it is
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accessed via SFTP or locally. The field ``chunker_polynomial`` contains a
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parameter that is used for splitting large files into smaller chunks (see
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below).
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Repository Layout
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-----------------
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@ -186,40 +189,75 @@ After decryption, a Pack's header consists of the following elements:
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::
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Type_Blob1 || Length(EncryptedBlob1) || Hash(Plaintext_Blob1) ||
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Type_Blob1 || Data_Blob1 ||
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[...]
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Type_BlobN || Length(EncryptedBlobN) || Hash(Plaintext_Blobn) ||
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Type_BlobN || Data_BlobN ||
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The Blob type field is a single byte. What follows it depends on the type. The
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following Blob types are defined:
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+-----------+----------------------+-------------------------------------------------------------------------------+
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| Type | Meaning | Data |
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+===========+======================+===============================================================================+
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| 0b00 | data blob | ``Length(encrypted_blob) || Hash(plaintext_blob)`` |
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+-----------+----------------------+-------------------------------------------------------------------------------+
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| 0b01 | tree blob | ``Length(encrypted_blob) || Hash(plaintext_blob)`` |
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+-----------+----------------------+-------------------------------------------------------------------------------+
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| 0b10 | compressed data blob | ``Length(encrypted_blob) || Length(plaintext_blob) || Hash(plaintext_blob)`` |
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+-----------+----------------------+-------------------------------------------------------------------------------+
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| 0b11 | compressed tree blob | ``Length(encrypted_blob) || Length(plaintext_blob) || Hash(plaintext_blob)`` |
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+-----------+----------------------+-------------------------------------------------------------------------------+
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This is enough to calculate the offsets for all the Blobs in the Pack.
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Length is the length of a Blob as a four byte integer in little-endian
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format. The type field is a one byte field and labels the content of a
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blob according to the following table:
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The length fields are encoded as four byte integers in little-endian
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format. In the Data column, ``Length(plaintext_blob)`` means the length
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of the decrypted and uncompressed data a blob consists of.
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+--------+-----------+
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| Type | Meaning |
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+========+===========+
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| 0 | data |
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+--------+-----------+
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| 1 | tree |
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+--------+-----------+
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All other types are invalid, more types may be added in the future. The
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compressed types are only valid for repository format version 2. Data and
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tree blobs may be compressed with the zstandard compression algorithm.
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All other types are invalid, more types may be added in the future.
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In repository format version 1, data and tree blobs should be stored in
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separate pack files. In version 2, they must be stored in separate files.
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Compressed and non-compress blobs of the same type may be mixed in a pack
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file.
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For reconstructing the index or parsing a pack without an index, first
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the last four bytes must be read in order to find the length of the
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header. Afterwards, the header can be read and parsed, which yields all
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plaintext hashes, types, offsets and lengths of all included blobs.
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Unpacked Data Format
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====================
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Individual files for the index, locks or snapshots are encrypted
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and authenticated like Data and Tree Blobs, so the outer structure is
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``IV || Ciphertext || MAC`` again. In repository format version 1 the
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plaintext always consists of a JSON document which must either be an
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object or an array.
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Repository format version 2 adds support for compression. The plaintext
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now starts with a header to indicate the encoding version to distinguish
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it from plain JSON and to allow for further evolution of the storage format:
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``encoding_version || data``
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The ``encoding_version`` field is encoded as one byte.
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For backwards compatibility the encoding versions '[' (0x5b) and '{' (0x7b)
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are used to mark that the whole plaintext (including the encoding version
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byte) should treated as JSON document.
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For new data the encoding version is currently always ``2``. For that
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version ``data`` contains a JSON document compressed using the zstandard
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compression algorithm.
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Indexing
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========
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Index files contain information about Data and Tree Blobs and the Packs
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they are contained in and store this information in the repository. When
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the local cached index is not accessible any more, the index files can
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be downloaded and used to reconstruct the index. The files are encrypted
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and authenticated like Data and Tree Blobs, so the outer structure is
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``IV || Ciphertext || MAC`` again. The plaintext consists of a JSON
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document like the following:
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be downloaded and used to reconstruct the index. The file encoding is
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described in the "Unpacked Data Format" section. The plaintext consists
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of a JSON document like the following:
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.. code:: json
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@ -235,18 +273,22 @@ document like the following:
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"id": "3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce",
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"type": "data",
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"offset": 0,
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"length": 25
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},{
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"length": 38,
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// no 'uncompressed_length' as blob is not compressed
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},
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{
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"id": "9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae",
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"type": "tree",
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"offset": 38,
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"length": 100
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"length": 112,
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"uncompressed_length": 511,
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},
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{
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"id": "d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66",
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"type": "data",
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"offset": 150,
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"length": 123
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"length": 123,
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"uncompressed_length": 234,
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}
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]
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}, [...]
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@ -255,7 +297,11 @@ document like the following:
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This JSON document lists Packs and the blobs contained therein. In this
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example, the Pack ``73d04e61`` contains two data Blobs and one Tree
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blob, the plaintext hashes are listed afterwards.
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blob, the plaintext hashes are listed afterwards. The ``length`` field
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corresponds to ``Length(encrypted_blob)`` in the pack file header.
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Field ``uncompressed_length`` is only present for compressed blobs and
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therefore is never present in version 1. It is set to the value of
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``Length(blob)``.
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The field ``supersedes`` lists the storage IDs of index files that have
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been replaced with the current index file. This happens when index files
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@ -350,8 +396,9 @@ Snapshots
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A snapshot represents a directory with all files and sub-directories at
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a given point in time. For each backup that is made, a new snapshot is
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created. A snapshot is a JSON document that is stored in an encrypted
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file below the directory ``snapshots`` in the repository. The filename
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created. A snapshot is a JSON document that is stored in a file below
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the directory ``snapshots`` in the repository. It uses the file encoding
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described in the "Unpacked Data Format" section. The filename
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is the storage ID. This string is unique and used within restic to
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uniquely identify a snapshot.
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@ -517,8 +564,8 @@ time there must not be any other locks (exclusive and non-exclusive).
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There may be multiple non-exclusive locks in parallel.
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A lock is a file in the subdir ``locks`` whose filename is the storage
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ID of the contents. It is encrypted and authenticated the same way as
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other files in the repository and contains the following JSON structure:
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ID of the contents. It is stored in the file encoding described in the
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"Unpacked Data Format" section and contains the following JSON structure:
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.. code:: json
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@ -721,3 +768,11 @@ An adversary who has a leaked (decrypted) key for a repository could:
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only be done using the ``copy`` command, which moves the data into a new
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repository with a new master key, or by making a completely new repository
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and new backup.
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Changes
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=======
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Repository Version 2
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--------------------
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* Support compression for blobs (data/tree) and index / lock / snapshot files
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