restic/doc/Design.md

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 at a
repository in structured form, for example in a file system hierarchy of with
several subdirectories. A repository implementation must be able to fulfil a
number of operations, e.g. list the contents.

*Blob*: A Blob combines a number of data bytes with identifying information
like the SHA256 hash of the data and its length.

*Pack*: A Pack combines one or more Blobs together, 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 needed 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. The directory layout is the same for
both access methods. This repository type is described in the following.

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 easily checking all 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 below 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:

    [ {
      "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 Blobs with contents. In this example, the Pack
`73d04e61` contains two data Blobs and one Tree blob, the plaintext hashes are
listed afterwards.

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`. They key `r` is then masked for use with Poly1305 (see the
paper for details).

This message authentication key is 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 are saved in a subdirectory of the
directory `trees`. The sub directory's name is the first two characters of the
filename the tree object is stored in.

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.

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 meta data should not
   be possible without a password for the repository. Everything except the
   `version` and `id` files and the meta data 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.