mirror of
https://github.com/octoleo/restic.git
synced 2024-11-26 23:06:32 +00:00
490 lines
21 KiB
Markdown
490 lines
21 KiB
Markdown
This document gives a high-level overview of the design and repository layout
|
|
of the restic backup program.
|
|
|
|
Terminology
|
|
===========
|
|
|
|
This section introduces terminology used in this document.
|
|
|
|
*Repository*: All data produced during a backup is sent to and stored in a
|
|
repository in a structured form, for example in a file system hierarchy with
|
|
several subdirectories. A repository implementation must be able to fulfill a
|
|
number of operations, e.g. list the contents.
|
|
|
|
*Blob*: A Blob combines a number of data bytes with identifying information
|
|
like the SHA-256 hash of the data and its length.
|
|
|
|
*Pack*: A Pack combines one or more Blobs, e.g. in a single file.
|
|
|
|
*Snapshot*: A Snapshot stands for the state of a file or directory that has
|
|
been backed up at some point in time. The state here means the content and meta
|
|
data like the name and modification time for the file or the directory and its
|
|
contents.
|
|
|
|
*Storage ID*: A storage ID is the SHA-256 hash of the content stored in the
|
|
repository. This ID is required in order to load the file from the repository.
|
|
|
|
Repository Format
|
|
=================
|
|
|
|
All data is stored in a restic repository. A repository is able to store data
|
|
of several different types, which can later be requested based on an ID. This
|
|
so-called "storage ID" is the SHA-256 hash of the content of a file. All files
|
|
in a repository are only written once and never modified afterwards. This
|
|
allows accessing and even writing to the repository with multiple clients in
|
|
parallel. Only the delete operation removes data from the repository.
|
|
|
|
At the time of writing, the only implemented repository type is based on
|
|
directories and files. Such repositories can be accessed locally on the same
|
|
system or via the integrated SFTP client (or any other storage back end).
|
|
The directory layout is the same for both access methods.
|
|
This repository type is described in the following section.
|
|
|
|
Repositories consist of several directories and a file called `config`. For
|
|
all other files stored in the repository, the name for the file is the lower
|
|
case hexadecimal representation of the storage ID, which is the SHA-256 hash of
|
|
the file's contents. This allows for easy verification of files for accidental
|
|
modifications, like disk read errors, by simply running the program `sha256sum`
|
|
and comparing its output to the file name. If the prefix of a filename is
|
|
unique amongst all the other files in the same directory, the prefix may be
|
|
used instead of the complete filename.
|
|
|
|
Apart from the files stored within the `keys` directory, all files are encrypted
|
|
with AES-256 in counter mode (CTR). The integrity of the encrypted data is
|
|
secured by a Poly1305-AES message authentication code (sometimes also referred
|
|
to as a "signature").
|
|
|
|
In the first 16 bytes of each encrypted file the initialisation vector (IV) is
|
|
stored. It is followed by the encrypted data and completed by the 16 byte
|
|
MAC. The format is: `IV || CIPHERTEXT || MAC`. The complete encryption
|
|
overhead is 32 bytes. For each file, a new random IV is selected.
|
|
|
|
The file `config` is encrypted this way and contains a JSON document like the
|
|
following:
|
|
|
|
{
|
|
"version": 1,
|
|
"id": "5956a3f67a6230d4a92cefb29529f10196c7d92582ec305fd71ff6d331d6271b",
|
|
"chunker_polynomial": "25b468838dcb75"
|
|
}
|
|
|
|
After decryption, restic first checks that the version field contains a version
|
|
number that it understands, otherwise it aborts. At the moment, the version is
|
|
expected to be 1. The field `id` holds a unique ID which consists of 32
|
|
random bytes, encoded in hexadecimal. This uniquely identifies the repository,
|
|
regardless if it is accessed via SFTP or locally. The field
|
|
`chunker_polynomial` contains a parameter that is used for splitting large
|
|
files into smaller chunks (see below).
|
|
|
|
The basic layout of a sample restic repository is shown here:
|
|
|
|
/tmp/restic-repo
|
|
├── config
|
|
├── data
|
|
│ ├── 21
|
|
│ │ └── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
|
|
│ ├── 32
|
|
│ │ └── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
|
|
│ ├── 59
|
|
│ │ └── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
|
|
│ ├── 73
|
|
│ │ └── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
|
|
│ [...]
|
|
├── index
|
|
│ ├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
|
|
│ └── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
|
|
├── keys
|
|
│ └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
|
|
├── locks
|
|
├── snapshots
|
|
│ └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
|
|
└── tmp
|
|
|
|
A repository can be initialized with the `restic init` command, e.g.:
|
|
|
|
$ restic -r /tmp/restic-repo init
|
|
|
|
Pack Format
|
|
-----------
|
|
|
|
All files in the repository except Key and Pack files just contain raw data,
|
|
stored as `IV || Ciphertext || MAC`. Pack files may contain one or more Blobs
|
|
of data.
|
|
|
|
A Pack's structure is as follows:
|
|
|
|
EncryptedBlob1 || ... || EncryptedBlobN || EncryptedHeader || Header_Length
|
|
|
|
At the end of the Pack file is a header, which describes the content. The
|
|
header is encrypted and authenticated. `Header_Length` is the length of the
|
|
encrypted header encoded as a four byte integer in little-endian encoding.
|
|
Placing the header at the end of a file allows writing the blobs in a
|
|
continuous stream as soon as they are read during the backup phase. This
|
|
reduces code complexity and avoids having to re-write a file once the pack is
|
|
complete and the content and length of the header is known.
|
|
|
|
All the blobs (`EncryptedBlob1`, `EncryptedBlobN` etc.) are authenticated and
|
|
encrypted independently. This enables repository reorganisation without having
|
|
to touch the encrypted Blobs. In addition it also allows efficient indexing,
|
|
for only the header needs to be read in order to find out which Blobs are
|
|
contained in the Pack. Since the header is authenticated, authenticity of the
|
|
header can be checked without having to read the complete Pack.
|
|
|
|
After decryption, a Pack's header consists of the following elements:
|
|
|
|
Type_Blob1 || Length(EncryptedBlob1) || Hash(Plaintext_Blob1) ||
|
|
[...]
|
|
Type_BlobN || Length(EncryptedBlobN) || Hash(Plaintext_Blobn) ||
|
|
|
|
This is enough to calculate the offsets for all the Blobs in the Pack. Length
|
|
is the length of a Blob as a four byte integer in little-endian format. The
|
|
type field is a one byte field and labels the content of a blob according to
|
|
the following table:
|
|
|
|
Type | Meaning
|
|
-----|---------
|
|
0 | data
|
|
1 | tree
|
|
|
|
All other types are invalid, more types may be added in the future.
|
|
|
|
For reconstructing the index or parsing a pack without an index, first the last
|
|
four bytes must be read in order to find the length of the header. Afterwards,
|
|
the header can be read and parsed, which yields all plaintext hashes, types,
|
|
offsets and lengths of all included blobs.
|
|
|
|
Indexing
|
|
--------
|
|
|
|
Index files contain information about Data and Tree Blobs and the Packs they
|
|
are contained in and store this information in the repository. When the local
|
|
cached index is not accessible any more, the index files can be downloaded and
|
|
used to reconstruct the index. The files are encrypted and authenticated like
|
|
Data and Tree Blobs, so the outer structure is `IV || Ciphertext || MAC` again.
|
|
The plaintext consists of a JSON document like the following:
|
|
|
|
{
|
|
"supersedes": [
|
|
"ed54ae36197f4745ebc4b54d10e0f623eaaaedd03013eb7ae90df881b7781452"
|
|
],
|
|
"packs": [
|
|
{
|
|
"id": "73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c",
|
|
"blobs": [
|
|
{
|
|
"id": "3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce",
|
|
"type": "data",
|
|
"offset": 0,
|
|
"length": 25
|
|
},{
|
|
"id": "9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae",
|
|
"type": "tree",
|
|
"offset": 38,
|
|
"length": 100
|
|
},
|
|
{
|
|
"id": "d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66",
|
|
"type": "data",
|
|
"offset": 150,
|
|
"length": 123
|
|
}
|
|
]
|
|
}, [...]
|
|
]
|
|
}
|
|
|
|
This JSON document lists Packs and the blobs contained therein. In this
|
|
example, the Pack `73d04e61` contains two data Blobs and one Tree blob, the
|
|
plaintext hashes are listed afterwards.
|
|
|
|
The field `supersedes` lists the storage IDs of index files that have been
|
|
replaced with the current index file. This happens when index files are
|
|
repacked, for example when old snapshots are removed and Packs are recombined.
|
|
|
|
There may be an arbitrary number of index files, containing information on
|
|
non-disjoint sets of Packs. The number of packs described in a single file is
|
|
chosen so that the file size is kept below 8 MiB.
|
|
|
|
Keys, Encryption and MAC
|
|
------------------------
|
|
|
|
All data stored by restic in the repository is encrypted with AES-256 in
|
|
counter mode and authenticated using Poly1305-AES. For encrypting new data first
|
|
16 bytes are read from a cryptographically secure pseudorandom number generator
|
|
as a random nonce. This is used both as the IV for counter mode and the nonce
|
|
for Poly1305. This operation needs three keys: A 32 byte for AES-256 for
|
|
encryption, a 16 byte AES key and a 16 byte key for Poly1305. For details see
|
|
the original paper [The Poly1305-AES message-authentication
|
|
code](http://cr.yp.to/mac/poly1305-20050329.pdf) by Dan Bernstein.
|
|
The data is then encrypted with AES-256 and afterwards a message authentication
|
|
code (MAC) is computed over the ciphertext, everything is then stored as
|
|
IV || CIPHERTEXT || MAC.
|
|
|
|
The directory `keys` contains key files. These are simple JSON documents which
|
|
contain all data that is needed to derive the repository's master encryption and
|
|
message authentication keys from a user's password. The JSON document from the
|
|
repository can be pretty-printed for example by using the Python module `json`
|
|
(shortened to increase readability):
|
|
|
|
$ python -mjson.tool /tmp/restic-repo/keys/b02de82*
|
|
{
|
|
"hostname": "kasimir",
|
|
"username": "fd0"
|
|
"kdf": "scrypt",
|
|
"N": 65536,
|
|
"r": 8,
|
|
"p": 1,
|
|
"created": "2015-01-02T18:10:13.48307196+01:00",
|
|
"data": "tGwYeKoM0C4j4/9DFrVEmMGAldvEn/+iKC3te/QE/6ox/V4qz58FUOgMa0Bb1cIJ6asrypCx/Ti/pRXCPHLDkIJbNYd2ybC+fLhFIJVLCvkMS+trdywsUkglUbTbi+7+Ldsul5jpAj9vTZ25ajDc+4FKtWEcCWL5ICAOoTAxnPgT+Lh8ByGQBH6KbdWabqamLzTRWxePFoYuxa7yXgmj9A==",
|
|
"salt": "uW4fEI1+IOzj7ED9mVor+yTSJFd68DGlGOeLgJELYsTU5ikhG/83/+jGd4KKAaQdSrsfzrdOhAMftTSih5Ux6w==",
|
|
}
|
|
|
|
When the repository is opened by restic, the user is prompted for the
|
|
repository password. This is then used with `scrypt`, a key derivation function
|
|
(KDF), and the supplied parameters (`N`, `r`, `p` and `salt`) to derive 64 key
|
|
bytes. The first 32 bytes are used as the encryption key (for AES-256) and the
|
|
last 32 bytes are used as the message authentication key (for Poly1305-AES).
|
|
These last 32 bytes are divided into a 16 byte AES key `k` followed by 16 bytes
|
|
of secret key `r`. The key `r` is then masked for use with Poly1305 (see the
|
|
paper for details).
|
|
|
|
Those message authentication keys (`k` and `r`) are used to compute a MAC over
|
|
the bytes contained in the JSON field `data` (after removing the Base64
|
|
encoding and not including the last 32 byte). If the password is incorrect or
|
|
the key file has been tampered with, the computed MAC will not match the last
|
|
16 bytes of the data, and restic exits with an error. Otherwise, the data is
|
|
decrypted with the encryption key derived from `scrypt`. This yields a JSON
|
|
document which contains the master encryption and message authentication keys
|
|
for this repository (encoded in Base64). The command `restic cat masterkey` can
|
|
be used as follows to decrypt and pretty-print the master key:
|
|
|
|
$ restic -r /tmp/restic-repo cat masterkey
|
|
{
|
|
"mac": {
|
|
"k": "evFWd9wWlndL9jc501268g==",
|
|
"r": "E9eEDnSJZgqwTOkDtOp+Dw=="
|
|
},
|
|
"encrypt": "UQCqa0lKZ94PygPxMRqkePTZnHRYh1k1pX2k2lM2v3Q=",
|
|
}
|
|
|
|
All data in the repository is encrypted and authenticated with these master keys.
|
|
For encryption, the AES-256 algorithm in Counter mode is used. For message
|
|
authentication, Poly1305-AES is used as described above.
|
|
|
|
A repository can have several different passwords, with a key file for each.
|
|
This way, the password can be changed without having to re-encrypt all data.
|
|
|
|
Snapshots
|
|
---------
|
|
|
|
A snapshots represents a directory with all files and sub-directories at a
|
|
given point in time. For each backup that is made, a new snapshot is created. A
|
|
snapshot is a JSON document that is stored in an encrypted file below the
|
|
directory `snapshots` in the repository. The filename is the storage ID. This
|
|
string is unique and used within restic to uniquely identify a snapshot.
|
|
|
|
The command `restic cat snapshot` can be used as follows to decrypt and
|
|
pretty-print the contents of a snapshot file:
|
|
|
|
$ restic -r /tmp/restic-repo cat snapshot 22a5af1b
|
|
enter password for repository:
|
|
{
|
|
"time": "2015-01-02T18:10:50.895208559+01:00",
|
|
"tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
|
|
"dir": "/tmp/testdata",
|
|
"hostname": "kasimir",
|
|
"username": "fd0",
|
|
"uid": 1000,
|
|
"gid": 100
|
|
}
|
|
|
|
Here it can be seen that this snapshot represents the contents of the directory
|
|
`/tmp/testdata`. The most important field is `tree`.
|
|
|
|
All content within a restic repository is referenced according to its SHA-256
|
|
hash. Before saving, each file is split into variable sized Blobs of data. The
|
|
SHA-256 hashes of all Blobs are saved in an ordered list which then represents
|
|
the content of the file.
|
|
|
|
In order to relate these plaintext hashes to the actual location within a Pack
|
|
file , an index is used. If the index is not available, the header of all data
|
|
Blobs can be read.
|
|
|
|
Trees and Data
|
|
--------------
|
|
|
|
A snapshot references a tree by the SHA-256 hash of the JSON string
|
|
representation of its contents. Trees and data are saved in pack files in a
|
|
subdirectory of the directory `data`.
|
|
|
|
The command `restic cat tree` can be used to inspect the tree referenced above:
|
|
|
|
$ restic -r /tmp/restic-repo cat tree b8138ab08a4722596ac89c917827358da4672eac68e3c03a8115b88dbf4bfb59
|
|
enter password for repository:
|
|
{
|
|
"nodes": [
|
|
{
|
|
"name": "testdata",
|
|
"type": "dir",
|
|
"mode": 493,
|
|
"mtime": "2014-12-22T14:47:59.912418701+01:00",
|
|
"atime": "2014-12-06T17:49:21.748468803+01:00",
|
|
"ctime": "2014-12-22T14:47:59.912418701+01:00",
|
|
"uid": 1000,
|
|
"gid": 100,
|
|
"user": "fd0",
|
|
"inode": 409704562,
|
|
"content": null,
|
|
"subtree": "b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc"
|
|
}
|
|
]
|
|
}
|
|
|
|
A tree contains a list of entries (in the field `nodes`) which contain meta
|
|
data like a name and timestamps. When the entry references a directory, the
|
|
field `subtree` contains the plain text ID of another tree object.
|
|
|
|
When the command `restic cat tree` is used, the storage hash is needed to print
|
|
a tree. The tree referenced above can be dumped as follows:
|
|
|
|
$ restic -r /tmp/restic-repo cat tree 8b238c8811cc362693e91a857460c78d3acf7d9edb2f111048691976803cf16e
|
|
enter password for repository:
|
|
{
|
|
"nodes": [
|
|
{
|
|
"name": "testfile",
|
|
"type": "file",
|
|
"mode": 420,
|
|
"mtime": "2014-12-06T17:50:23.34513538+01:00",
|
|
"atime": "2014-12-06T17:50:23.338468713+01:00",
|
|
"ctime": "2014-12-06T17:50:23.34513538+01:00",
|
|
"uid": 1000,
|
|
"gid": 100,
|
|
"user": "fd0",
|
|
"inode": 416863351,
|
|
"size": 1234,
|
|
"links": 1,
|
|
"content": [
|
|
"50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d"
|
|
]
|
|
},
|
|
[...]
|
|
]
|
|
}
|
|
|
|
This tree contains a file entry. This time, the `subtree` field is not present
|
|
and the `content` field contains a list with one plain text SHA-256 hash.
|
|
|
|
The command `restic cat data` can be used to extract and decrypt data given a
|
|
plaintext ID, e.g. for the data mentioned above:
|
|
|
|
$ restic -r /tmp/restic-repo cat blob 50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d | sha256sum
|
|
enter password for repository:
|
|
50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d -
|
|
|
|
As can be seen from the output of the program `sha256sum`, the hash matches the
|
|
plaintext hash from the map included in the tree above, so the correct data has
|
|
been returned.
|
|
|
|
Locks
|
|
-----
|
|
|
|
The restic repository structure is designed in a way that allows parallel
|
|
access of multiple instance of restic and even parallel writes. However, there
|
|
are some functions that work more efficient or even require exclusive access of
|
|
the repository. In order to implement these functions, restic processes are
|
|
required to create a lock on the repository before doing anything.
|
|
|
|
Locks come in two types: Exclusive and non-exclusive locks. At most one
|
|
process can have an exclusive lock on the repository, and during that time
|
|
there must not be any other locks (exclusive and non-exclusive). There may be
|
|
multiple non-exclusive locks in parallel.
|
|
|
|
A lock is a file in the subdir `locks` whose filename is the storage ID of
|
|
the contents. It is encrypted and authenticated the same way as other files
|
|
in the repository and contains the following JSON structure:
|
|
|
|
{
|
|
"time": "2015-06-27T12:18:51.759239612+02:00",
|
|
"exclusive": false,
|
|
"hostname": "kasimir",
|
|
"username": "fd0",
|
|
"pid": 13607,
|
|
"uid": 1000,
|
|
"gid": 100
|
|
}
|
|
|
|
The field `exclusive` defines the type of lock. When a new lock is to be
|
|
created, restic checks all locks in the repository. When a lock is found, it
|
|
is tested if the lock is stale, which is the case for locks with timestamps
|
|
older than 30 minutes. If the lock was created on the same machine, even for
|
|
younger locks it is tested whether the process is still alive by sending a
|
|
signal to it. If that fails, restic assumes that the process is dead and
|
|
considers the lock to be stale.
|
|
|
|
When a new lock is to be created and no other conflicting locks are
|
|
detected, restic creates a new lock, waits, and checks if other locks
|
|
appeared in the repository. Depending on the type of the other locks and the
|
|
lock to be created, restic either continues or fails.
|
|
|
|
Backups and Deduplication
|
|
=========================
|
|
|
|
For creating a backup, restic scans the source directory for all files,
|
|
sub-directories and other entries. The data from each file is split into
|
|
variable length Blobs cut at offsets defined by a sliding window of 64 byte.
|
|
The implementation uses Rabin Fingerprints for implementing this Content
|
|
Defined Chunking (CDC). An irreducible polynomial is selected at random and
|
|
saved in the file `config` when a repository is initialized, so that watermark
|
|
attacks are much harder.
|
|
|
|
Files smaller than 512 KiB are not split, Blobs are of 512 KiB to 8 MiB in
|
|
size. The implementation aims for 1 MiB Blob size on average.
|
|
|
|
For modified files, only modified Blobs have to be saved in a subsequent
|
|
backup. This even works if bytes are inserted or removed at arbitrary positions
|
|
within the file.
|
|
|
|
Threat Model
|
|
============
|
|
|
|
The design goals for restic include being able to securely store backups in a
|
|
location that is not completely trusted, e.g. a shared system where others can
|
|
potentially access the files or (in the case of the system administrator) even
|
|
modify or delete them.
|
|
|
|
General assumptions:
|
|
|
|
* The host system a backup is created on is trusted. This is the most basic
|
|
requirement, and essential for creating trustworthy backups.
|
|
|
|
The restic backup program guarantees the following:
|
|
|
|
* Accessing the unencrypted content of stored files and metadata should not
|
|
be possible without a password for the repository. Everything except the
|
|
metadata included for informational purposes in the key files is encrypted and
|
|
authenticated.
|
|
|
|
* Modifications (intentional or unintentional) can be detected automatically
|
|
on several layers:
|
|
|
|
1. For all accesses of data stored in the repository it is checked whether
|
|
the cryptographic hash of the contents matches the storage ID (the
|
|
file's name). This way, modifications (bad RAM, broken harddisk) can be
|
|
detected easily.
|
|
|
|
2. Before decrypting any data, the MAC on the encrypted data is
|
|
checked. If there has been a modification, the MAC check will
|
|
fail. This step happens even before the data is decrypted, so data that
|
|
has been tampered with is not decrypted at all.
|
|
|
|
However, the restic backup program is not designed to protect against attackers
|
|
deleting files at the storage location. There is nothing that can be done about
|
|
this. If this needs to be guaranteed, get a secure location without any access
|
|
from third parties. If you assume that attackers have write access to your
|
|
files at the storage location, attackers are able to figure out (e.g. based on
|
|
the timestamps of the stored files) which files belong to what snapshot. When
|
|
only these files are deleted, the particular snapshot vanished and all
|
|
snapshots depending on data that has been added in the snapshot cannot be
|
|
restored completely. Restic is not designed to detect this attack.
|