Alexander Neumann 5e721afb5d doc/mkdocs: Improve code hilighting

Additionally, refresh the restic sample output.

2016-11-08 20:23:39 +01:00

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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 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.

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