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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. Writing should occur atomically to prevent concurrent operations from reading incomplete files. This allows accessing and even writing to the repository with multiple clients in parallel. Only the prune operation removes data from the repository.

Repositories consist of several directories and a top-level 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 on the file 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 and data directories, 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 (MAC). Files in the data directory ("pack files") consist of multiple parts which are all independently encrypted and authenticated, see below.

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": 2,
  "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 or 2. The list of changes in the repository format is contained in the section "Changes" below.

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 a remote storage backend or locally. The field chunker_polynomial contains a parameter that is used for splitting large files into smaller chunks (see below).

Repository Layout

The local and sftp backends are implemented using files and directories stored in a file system. The directory layout is the same for both backend types and is also used for all other remote backends.

The basic layout of a repository is shown here:

├── config
├── data
│   ├── 21
│   │   └── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
│   ├── 32
│   │   └── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
│   ├── 59
│   │   └── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
│   ├── 73
│   │   └── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
│   [...]
├── index
│   ├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
│   └── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
├── keys
│   └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
├── locks
├── snapshots
│   └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
└── tmp

A local repository can be initialized with the restic init command, e.g.:

$ restic -r /tmp/restic-repo init

The local and sftp backends will auto-detect and accept all layouts described in the following sections, so that remote repositories mounted locally e.g. via fuse can be accessed. The layout auto-detection can be overridden by specifying the option -o local.layout=default, valid values are default and s3legacy. The option for the sftp backend is named sftp.layout, for the s3 backend s3.layout.

S3 Legacy Layout

Unfortunately during development the Amazon S3 backend uses slightly different paths (directory names use singular instead of plural for key, lock, and snapshot files), and the pack files are stored directly below the data directory. The S3 Legacy repository layout looks like this:

 ├── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
 ├── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
 ├── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
 ├── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
 ├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
 └── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
 └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
 └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec

The S3 backend understands and accepts both forms, new backends are always created with the default layout for compatibility reasons.

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 || Data_Blob1 ||
Type_BlobN || Data_BlobN ||

The Blob type field is a single byte. What follows it depends on the type. The following Blob types are defined:

Type Meaning Data
0b00 data blob Length(encrypted_blob) || Hash(plaintext_blob)
0b01 tree blob Length(encrypted_blob) || Hash(plaintext_blob)
0b10 compressed data blob Length(encrypted_blob) || Length(plaintext_blob) || Hash(plaintext_blob)
0b11 compressed tree blob Length(encrypted_blob) || Length(plaintext_blob) || Hash(plaintext_blob)

This is enough to calculate the offsets for all the Blobs in the Pack. The length fields are encoded as four byte integers in little-endian format. In the Data column, Length(plaintext_blob) means the length of the decrypted and uncompressed data a blob consists of.

All other types are invalid, more types may be added in the future. The compressed types are only valid for repository format version 2. Data and tree blobs may be compressed with the zstandard compression algorithm.

In repository format version 1, data and tree blobs should be stored in separate pack files. In version 2, they must be stored in separate files. Compressed and non-compress blobs of the same type may be mixed in a pack file.

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.

Unpacked Data Format

Individual files for the index, locks or snapshots are encrypted and authenticated like Data and Tree Blobs, so the outer structure is IV || Ciphertext || MAC again. In repository format version 1 the plaintext always consists of a JSON document which must either be an object or an array.

Repository format version 2 adds support for compression. The plaintext now starts with a header to indicate the encoding version to distinguish it from plain JSON and to allow for further evolution of the storage format: encoding_version || data The encoding_version field is encoded as one byte. For backwards compatibility the encoding versions '[' (0x5b) and '{' (0x7b) are used to mark that the whole plaintext (including the encoding version byte) should treated as JSON document.

For new data the encoding version is currently always 2. For that version data contains a JSON document compressed using the zstandard compression algorithm.


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 file encoding is described in the "Unpacked Data Format" section. The plaintext consists of a JSON document like the following:

  "supersedes": [
  "packs": [
      "id": "73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c",
      "blobs": [
          "id": "3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce",
          "type": "data",
          "offset": 0,
          "length": 38,
          // no 'uncompressed_length' as blob is not compressed
          "id": "9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae",
          "type": "data",
          "offset": 38,
          "length": 112,
          "uncompressed_length": 511,
          "id": "d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66",
          "type": "data",
          "offset": 150,
          "length": 123,
          "uncompressed_length": 234,
    }, [...]

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 length field corresponds to Length(encrypted_blob) in the pack file header. Field uncompressed_length is only present for compressed blobs and therefore is never present in version 1 of the repository format. It is set to the value of Length(blob).

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 pseudo-random 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 keys are used to authenticate and decrypt the bytes contained in the JSON field data with AES-256 and Poly1305-AES as if they were any other blob (after removing the Base64 encoding). 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 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.


A snapshot 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 a file below the directory snapshots in the repository. It uses the file encoding described in the "Unpacked Data Format" section. 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 251c2e58
enter password for repository:
  "time": "2015-01-02T18:10:50.895208559+01:00",
  "tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
  "paths": [
  "hostname": "kasimir",
  "username": "fd0",
  "uid": 1000,
  "gid": 100,
  "tags": [

Here it can be seen that this snapshot represents the contents of the directory /tmp/testdata. The most important field is tree. When the meta data (e.g. the tags) of a snapshot change, the snapshot needs to be re-encrypted and saved. This will change the storage ID, so in order to relate these seemingly different snapshots, a field original is introduced which contains the ID of the original snapshot, e.g. after adding the tag DE to the snapshot above it becomes:

$ restic -r /tmp/restic-repo cat snapshot 22a5af1b
enter password for repository:
  "time": "2015-01-02T18:10:50.895208559+01:00",
  "tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
  "paths": [
  "hostname": "kasimir",
  "username": "fd0",
  "uid": 1000,
  "gid": 100,
  "tags": [
  "original": "251c2e5841355f743f9d4ffd3260bee765acee40a6229857e32b60446991b837"

Once introduced, the original field is not modified when the snapshot's meta data is changed again.

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 blob can be used to inspect the tree referenced above (piping the output of the command to jq . so that the JSON is indented):

$ restic -r /tmp/restic-repo cat blob 2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf | jq .
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. Note that there are some specialties of how this metadata is generated:

  • The name is quoted using strconv.Quote before being saved. This handles non-unicode names, but also changes the representation of names containing " or \.
  • The filemode saved is the mode defined by fs.FileMode masked by os.ModePerm | os.ModeType | os.ModeSetuid | os.ModeSetgid | os.ModeSticky

When the entry references a directory, the field subtree contains the plain text ID of another tree object.

When the command restic cat blob is used, the plaintext ID is needed to print a tree. The tree referenced above can be dumped as follows:

$ restic -r /tmp/restic-repo cat blob b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc | jq .
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": [

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.

A symlink uses the following data structure:

$ restic -r /tmp/restic-repo cat blob 4c0a7d500bd1482ba01752e77c8d5a923304777d96b6522fae7c11e99b4e6fa6 | jq .
enter password for repository:
  "nodes": [
      "name": "testlink",
      "type": "symlink",
      "mode": 134218239,
      "mtime": "2023-07-25T20:01:44.007465374+02:00",
      "atime": "2023-07-25T20:01:44.007465374+02:00",
      "ctime": "2023-07-25T20:01:44.007465374+02:00",
      "uid": 1000,
      "gid": 100,
      "user": "fd0",
      "inode": 33734827,
      "links": 1,
      "linktarget": "example_target",
      "content": null

The symlink target is stored in the field linktarget. As JSON strings can only contain valid unicode, an exception applies if the linktarget is not a valid UTF-8 string. Since restic 0.16.0, in such a case the linktarget_raw field contains a base64 encoded version of the raw linktarget. The linktarget_raw field is only set if linktarget cannot be encoded correctly.

The command restic cat blob can also 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.


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 stored in the file encoding described in the "Unpacked Data Format" section 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. If the --retry-lock option is specified, restic will retry creating the lock periodically until it succeeds or the specified timeout expires.

Read and Write Ordering

The repository format allows writing (e.g. backup) and reading (e.g. restore) to happen concurrently. As the data for each snapshot in a repository spans multiple files (snapshot, index and packs), it is necessary to follow certain rules regarding the order in which files are read and written. These ordering rules also guarantee that repository modifications always maintain a correct repository even if the client or the storage backend crashes for example due to a power cut or the (network) connection between both is interrupted.

The correct order to access data in a repository is derived from the following set of invariants that must be maintained at any time in a correct repository. Must in the following is a strict requirement and will lead to data loss if not followed. Should will require steps to fix a repository (e.g. rebuilding the index) if not followed, but should not cause data loss. existing means that the referenced data is durably stored in the repository.

  • A snapshot must only reference an existing tree blob.
  • A reachable tree blob must only reference tree and data blobs that exist (recursively). Reachable means that the tree blob is reachable starting from a snapshot.
  • An index must only reference valid blobs in existing packs.
  • All blobs referenced by a snapshot should be listed in an index.

This leads to the following recommended order to store data in a repository. First, pack files, which contain data and tree blobs, must be written. Then the indexes which reference blobs in these already written pack files. And finally the corresponding snapshots.

Note that there is no need for a specific write order of data and tree blobs during a backup as the blobs only become referenced once the corresponding snapshot is uploaded.

Reading data should follow the opposite order compared to writing. Only once a snapshot was written, it is guaranteed that all required data exists in the repository. This especially means that the list of snapshots to read should be collected before loading the repository index. The other way round can lead to a race condition where a recently written snapshot is loaded but not its accompanying index, which results in a failure to access the snapshot's tree blob.

For removing or rewriting data from a repository the following rules must be followed, which are derived from the above invariants.

  • A client removing data must acquire an exclusive lock first to prevent conflicts with other clients.
  • A pack must be removed from the referencing index before it is deleted.
  • Rewriting a pack must write the new pack, update the index (add an updated index and delete the old one) and only then delete the old pack.

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 bytes. 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 even modify or delete them in the case of the system administrator.

General assumptions:

  • The host system a backup is created on is trusted. This is the most basic requirement, and it is essential for creating trustworthy backups.
  • The user uses an authentic copy of restic.
  • The user does not share the repository password with an attacker.
  • 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.
  • The whole repository is re-encrypted if a key is leaked. With the current key management design, it is impossible to securely revoke a leaked key without re-encrypting the whole repository.
  • Advances in cryptography attacks against the cryptographic primitives used by restic (i.e., AES-256-CTR-Poly1305-AES and SHA-256) have not occurred. Such advances could render the confidentiality or integrity protections provided by restic useless.
  • Sufficient advances in computing have not occurred to make brute-force attacks against restic's cryptographic protections feasible.

The restic backup program guarantees the following:

  • Unencrypted content of stored files and metadata cannot be accessed without a password for the repository. Everything except the metadata included for informational purposes in the key files is encrypted and authenticated. The cache is also encrypted to prevent metadata leaks.
  • Modifications to data stored in the repository (due to bad RAM, broken harddisk, etc.) can be detected.
  • Data that has been tampered will not be decrypted.

With the aforementioned assumptions and guarantees in mind, the following are examples of things an adversary could achieve in various circumstances.

An adversary with read access to your backup storage location could:

  • Attempt a brute force password guessing attack against a copy of the repository (please use strong passwords with sufficient entropy).
  • Infer which packs probably contain trees via file access patterns.
  • Infer the size of backups by using creation timestamps of repository objects.

An adversary with network access could:

  • Attempt to DoS the server storing the backup repository or the network connection between client and server.
  • Determine from where you create your backups (i.e., the location where the requests originate).
  • Determine where you store your backups (i.e., which provider/target system).
  • Infer the size of backups by observing network traffic.

The following are examples of the implications associated with violating some of the aforementioned assumptions.

An adversary who compromises (via malware, physical access, etc.) the host system making backups could:

  • Render the entire backup process untrustworthy (e.g., intercept password, copy files, manipulate data).
  • Create snapshots (containing garbage data) which cover all modified files and wait until a trusted host has used forget often enough to remove all correct snapshots.
  • Create a garbage snapshot for every existing snapshot with a slightly different timestamp and wait until certain forget configurations have been run, thereby removing all correct snapshots at once.

An adversary with write access to your files at the storage location could:

  • Delete or manipulate your backups, thereby impairing your ability to restore files from the compromised storage location.
  • Determine which files belong to what snapshot (e.g., based on the timestamps of the stored files). When only these files are deleted, the particular snapshot vanishes 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.

An adversary who compromises a host system with append-only (read+write allowed, delete+overwrite denied) access to the backup repository could:

  • Capture the password and decrypt backups from the past and in the future (see the "leaked key" example below for related information).
  • Render new backups untrustworthy after the host has been compromised (due to having complete control over new backups). An attacker cannot delete or manipulate old backups. As such, restoring old snapshots created before a host compromise remains possible.
  • Potentially manipulate the use of the forget command into deleting all legitimate snapshots, keeping only bogus snapshots added by the attacker. Ransomware might try this in order to leave only one option to get your data back: paying the ransom. For safe use of forget, please see the corresponding documentation on removing backup snapshots and append-only mode.

An adversary who has a leaked (decrypted) key for a repository could:

  • Decrypt existing and future backup data. If multiple hosts backup into the same repository, an attacker will get access to the backup data of every host. Note that since the local encryption key gives access to the master key, a password change will not prevent this. Changing the master key can currently only be done using the copy command, which moves the data into a new repository with a new master key, or by making a completely new repository and new backup.


Repository Version 2

  • Support compression for blobs (data/tree) and index / lock / snapshot files