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438 lines
19 KiB
Markdown
438 lines
19 KiB
Markdown
This document gives a high-level overview of the design and repository layout
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of the restic backup program.
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Terminology
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===========
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This section introduces terminology used in this document.
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*Repository*: All data produced during a backup is sent to and stored at a
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repository in structured form, for example in a file system hierarchy of with
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several subdirectories. A repository implementation must be able to fulfil a
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number of operations, e.g. list the contents.
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*Blob*: A Blob combines a number of data bytes with identifying information
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like the SHA256 hash of the data and its length.
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*Pack*: A Pack combines one or more Blobs together, e.g. in a single file.
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*Snapshot*: A Snapshot stands for the state of a file or directory that has
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been backed up at some point in time. The state here means the content and meta
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data like the name and modification time for the file or the directory and its
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contents.
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*Storage ID*: A storage ID is the SHA-256 hash of the content stored in the
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repository. This ID is needed in order to load the file from the repository.
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Repository Format
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=================
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All data is stored in a restic repository. A repository is able to store data
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of several different types, which can later be requested based on an ID. This
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so-called "storage ID" is the SHA-256 hash of the content of a file. All files
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in a repository are only written once and never modified afterwards. This
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allows accessing and even writing to the repository with multiple clients in
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parallel. Only the delete operation removes data from the repository.
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At the time of writing, the only implemented repository type is based on
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directories and files. Such repositories can be accessed locally on the same
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system or via the integrated SFTP client. The directory layout is the same for
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both access methods. This repository type is described in the following.
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Repositories consist of several directories and a file called `config`. For
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all other files stored in the repository, the name for the file is the lower
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case hexadecimal representation of the storage ID, which is the SHA-256 hash of
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the file's contents. This allows easily checking all files for accidental
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modifications like disk read errors by simply running the program `sha256sum`
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and comparing its output to the file name. If the prefix of a filename is
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unique amongst all the other files in the same directory, the prefix may be
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used instead of the complete filename.
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Apart from the files stored below the `keys` directory, all files are encrypted
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with AES-256 in counter mode (CTR). The integrity of the encrypted data is
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secured by a Poly1305-AES message authentication code (sometimes also referred
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to as a "signature").
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In the first 16 bytes of each encrypted file the initialisation vector (IV) is
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stored. It is followed by the encrypted data and completed by the 16 byte
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MAC. The format is: `IV || CIPHERTEXT || MAC`. The complete encryption
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overhead is 32 bytes. For each file, a new random IV is selected.
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The file `config` is encrypted this way and contains a JSON document like the
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following:
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{
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"version": 1,
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"id": "5956a3f67a6230d4a92cefb29529f10196c7d92582ec305fd71ff6d331d6271b",
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"chunker_polynomial": "25b468838dcb75"
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}
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After decryption, restic first checks that the version field contains a version
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number that it understands, otherwise it aborts. At the moment, the version is
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expected to be 1. The field `id` holds a unique ID which consists of 32
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random bytes, encoded in hexadecimal. This uniquely identifies the repository,
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regardless if it is accessed via SFTP or locally. The field
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`chunker_polynomial` contains a parameter that is used for splitting large
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files into smaller chunks (see below).
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The basic layout of a sample restic repository is shown here:
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/tmp/restic-repo
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├── config
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├── data
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│ ├── 21
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│ │ └── 2159dd48f8a24f33c307b750592773f8b71ff8d11452132a7b2e2a6a01611be1
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│ ├── 32
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│ │ └── 32ea976bc30771cebad8285cd99120ac8786f9ffd42141d452458089985043a5
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│ ├── 59
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│ │ └── 59fe4bcde59bd6222eba87795e35a90d82cd2f138a27b6835032b7b58173a426
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│ ├── 73
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│ │ └── 73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c
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│ [...]
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├── index
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│ ├── c38f5fb68307c6a3e3aa945d556e325dc38f5fb68307c6a3e3aa945d556e325d
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│ └── ca171b1b7394d90d330b265d90f506f9984043b342525f019788f97e745c71fd
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├── keys
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│ └── b02de829beeb3c01a63e6b25cbd421a98fef144f03b9a02e46eff9e2ca3f0bd7
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├── locks
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├── snapshots
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│ └── 22a5af1bdc6e616f8a29579458c49627e01b32210d09adb288d1ecda7c5711ec
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└── tmp
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A repository can be initialized with the `restic init` command, e.g.:
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$ restic -r /tmp/restic-repo init
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Pack Format
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-----------
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All files in the repository except Key and Pack files just contain raw data,
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stored as `IV || Ciphertext || MAC`. Pack files may contain one or more Blobs
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of data.
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A Pack's structure is as follows:
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EncryptedBlob1 || ... || EncryptedBlobN || EncryptedHeader || Header_Length
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At the end of the Pack file is a header, which describes the content. The
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header is encrypted and authenticated. `Header_Length` is the length of the
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encrypted header encoded as a four byte integer in little-endian encoding.
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Placing the header at the end of a file allows writing the blobs in a
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continuous stream as soon as they are read during the backup phase. This
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reduces code complexity and avoids having to re-write a file once the pack is
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complete and the content and length of the header is known.
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All the blobs (`EncryptedBlob1`, `EncryptedBlobN` etc.) are authenticated and
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encrypted independently. This enables repository reorganisation without having
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to touch the encrypted Blobs. In addition it also allows efficient indexing,
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for only the header needs to be read in order to find out which Blobs are
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contained in the Pack. Since the header is authenticated, authenticity of the
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header can be checked without having to read the complete Pack.
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After decryption, a Pack's header consists of the following elements:
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Type_Blob1 || Length(EncryptedBlob1) || Hash(Plaintext_Blob1) ||
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[...]
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Type_BlobN || Length(EncryptedBlobN) || Hash(Plaintext_Blobn) ||
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This is enough to calculate the offsets for all the Blobs in the Pack. Length
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is the length of a Blob as a four byte integer in little-endian format. The
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type field is a one byte field and labels the content of a blob according to
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the following table:
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Type | Meaning
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-----|---------
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0 | data
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1 | tree
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All other types are invalid, more types may be added in the future.
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For reconstructing the index or parsing a pack without an index, first the last
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four bytes must be read in order to find the length of the header. Afterwards,
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the header can be read and parsed, which yields all plaintext hashes, types,
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offsets and lengths of all included blobs.
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Indexing
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--------
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Index files contain information about Data and Tree Blobs and the Packs they
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are contained in and store this information in the repository. When the local
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cached index is not accessible any more, the index files can be downloaded and
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used to reconstruct the index. The files are encrypted and authenticated like
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Data and Tree Blobs, so the outer structure is `IV || Ciphertext || MAC` again.
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The plaintext consists of a JSON document like the following:
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[ {
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"id": "73d04e6125cf3c28a299cc2f3cca3b78ceac396e4fcf9575e34536b26782413c",
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"blobs": [
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{
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"id": "3ec79977ef0cf5de7b08cd12b874cd0f62bbaf7f07f3497a5b1bbcc8cb39b1ce",
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"type": "data",
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"offset": 0,
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"length": 25
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},{
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"id": "9ccb846e60d90d4eb915848add7aa7ea1e4bbabfc60e573db9f7bfb2789afbae",
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"type": "tree",
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"offset": 38,
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"length": 100
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},
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{
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"id": "d3dc577b4ffd38cc4b32122cabf8655a0223ed22edfd93b353dc0c3f2b0fdf66",
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"type": "data",
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"offset": 150,
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"length": 123
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}
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]
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} ]
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This JSON document lists Blobs with contents. In this example, the Pack
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`73d04e61` contains two data Blobs and one Tree blob, the plaintext hashes are
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listed afterwards.
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There may be an arbitrary number of index files, containing information on
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non-disjoint sets of Packs. The number of packs described in a single file is
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chosen so that the file size is kept below 8 MiB.
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Keys, Encryption and MAC
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------------------------
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All data stored by restic in the repository is encrypted with AES-256 in
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counter mode and authenticated using Poly1305-AES. For encrypting new data first
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16 bytes are read from a cryptographically secure pseudorandom number generator
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as a random nonce. This is used both as the IV for counter mode and the nonce
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for Poly1305. This operation needs three keys: A 32 byte for AES-256 for
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encryption, a 16 byte AES key and a 16 byte key for Poly1305. For details see
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the original paper [The Poly1305-AES message-authentication
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code](http://cr.yp.to/mac/poly1305-20050329.pdf) by Dan Bernstein.
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The data is then encrypted with AES-256 and afterwards a message authentication
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code (MAC) is computed over the ciphertext, everything is then stored as
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IV || CIPHERTEXT || MAC.
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The directory `keys` contains key files. These are simple JSON documents which
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contain all data that is needed to derive the repository's master encryption and
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message authentication keys from a user's password. The JSON document from the
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repository can be pretty-printed for example by using the Python module `json`
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(shortened to increase readability):
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$ python -mjson.tool /tmp/restic-repo/keys/b02de82*
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{
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"hostname": "kasimir",
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"username": "fd0"
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"kdf": "scrypt",
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"N": 65536,
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"r": 8,
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"p": 1,
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"created": "2015-01-02T18:10:13.48307196+01:00",
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"data": "tGwYeKoM0C4j4/9DFrVEmMGAldvEn/+iKC3te/QE/6ox/V4qz58FUOgMa0Bb1cIJ6asrypCx/Ti/pRXCPHLDkIJbNYd2ybC+fLhFIJVLCvkMS+trdywsUkglUbTbi+7+Ldsul5jpAj9vTZ25ajDc+4FKtWEcCWL5ICAOoTAxnPgT+Lh8ByGQBH6KbdWabqamLzTRWxePFoYuxa7yXgmj9A==",
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"salt": "uW4fEI1+IOzj7ED9mVor+yTSJFd68DGlGOeLgJELYsTU5ikhG/83/+jGd4KKAaQdSrsfzrdOhAMftTSih5Ux6w==",
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}
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When the repository is opened by restic, the user is prompted for the
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repository password. This is then used with `scrypt`, a key derivation function
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(KDF), and the supplied parameters (`N`, `r`, `p` and `salt`) to derive 64 key
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bytes. The first 32 bytes are used as the encryption key (for AES-256) and the
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last 32 bytes are used as the message authentication key (for Poly1305-AES).
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These last 32 bytes are divided into a 16 byte AES key `k` followed by 16 bytes
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of secret key `r`. They key `r` is then masked for use with Poly1305 (see the
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paper for details).
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This message authentication key is used to compute a MAC over the bytes contained
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in the JSON field `data` (after removing the Base64 encoding and not including
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the last 32 byte). If the password is incorrect or the key file has been
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tampered with, the computed MAC will not match the last 16 bytes of the data,
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and restic exits with an error. Otherwise, the data is decrypted with the
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encryption key derived from `scrypt`. This yields a JSON document which
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contains the master encryption and message authentication keys for this
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repository (encoded in Base64). The command `restic cat masterkey` can be used
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as follows to decrypt and pretty-print the master key:
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$ restic -r /tmp/restic-repo cat masterkey
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{
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"mac": {
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"k": "evFWd9wWlndL9jc501268g==",
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"r": "E9eEDnSJZgqwTOkDtOp+Dw=="
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},
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"encrypt": "UQCqa0lKZ94PygPxMRqkePTZnHRYh1k1pX2k2lM2v3Q=",
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}
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All data in the repository is encrypted and authenticated with these master keys.
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For encryption, the AES-256 algorithm in Counter mode is used. For message
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authentication, Poly1305-AES is used as described above.
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A repository can have several different passwords, with a key file for each.
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This way, the password can be changed without having to re-encrypt all data.
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Snapshots
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---------
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A snapshots represents a directory with all files and sub-directories at a
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given point in time. For each backup that is made, a new snapshot is created. A
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snapshot is a JSON document that is stored in an encrypted file below the
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directory `snapshots` in the repository. The filename is the storage ID. This
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string is unique and used within restic to uniquely identify a snapshot.
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The command `restic cat snapshot` can be used as follows to decrypt and
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pretty-print the contents of a snapshot file:
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$ restic -r /tmp/restic-repo cat snapshot 22a5af1b
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enter password for repository:
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{
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"time": "2015-01-02T18:10:50.895208559+01:00",
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"tree": "2da81727b6585232894cfbb8f8bdab8d1eccd3d8f7c92bc934d62e62e618ffdf",
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"dir": "/tmp/testdata",
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"hostname": "kasimir",
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"username": "fd0",
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"uid": 1000,
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"gid": 100
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}
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Here it can be seen that this snapshot represents the contents of the directory
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`/tmp/testdata`. The most important field is `tree`.
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All content within a restic repository is referenced according to its SHA-256
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hash. Before saving, each file is split into variable sized Blobs of data. The
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SHA-256 hashes of all Blobs are saved in an ordered list which then represents
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the content of the file.
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In order to relate these plaintext hashes to the actual location within a Pack
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file , an index is used. If the index is not available, the header of all data
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Blobs can be read.
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Trees and Data
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--------------
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A snapshot references a tree by the SHA-256 hash of the JSON string
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representation of its contents. Trees are saved in a subdirectory of the
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directory `trees`. The sub directory's name is the first two characters of the
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filename the tree object is stored in.
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The command `restic cat tree` can be used to inspect the tree referenced above:
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$ restic -r /tmp/restic-repo cat tree b8138ab08a4722596ac89c917827358da4672eac68e3c03a8115b88dbf4bfb59
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enter password for repository:
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{
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"nodes": [
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{
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"name": "testdata",
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"type": "dir",
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"mode": 493,
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"mtime": "2014-12-22T14:47:59.912418701+01:00",
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"atime": "2014-12-06T17:49:21.748468803+01:00",
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"ctime": "2014-12-22T14:47:59.912418701+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 409704562,
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"content": null,
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"subtree": "b26e315b0988ddcd1cee64c351d13a100fedbc9fdbb144a67d1b765ab280b4dc"
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}
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]
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}
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A tree contains a list of entries (in the field `nodes`) which contain meta
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data like a name and timestamps. When the entry references a directory, the
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field `subtree` contains the plain text ID of another tree object.
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When the command `restic cat tree` is used, the storage hash is needed to print
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a tree. The tree referenced above can be dumped as follows:
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$ restic -r /tmp/restic-repo cat tree 8b238c8811cc362693e91a857460c78d3acf7d9edb2f111048691976803cf16e
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enter password for repository:
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{
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"nodes": [
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{
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"name": "testfile",
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"type": "file",
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"mode": 420,
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"mtime": "2014-12-06T17:50:23.34513538+01:00",
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"atime": "2014-12-06T17:50:23.338468713+01:00",
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"ctime": "2014-12-06T17:50:23.34513538+01:00",
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"uid": 1000,
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"gid": 100,
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"user": "fd0",
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"inode": 416863351,
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"size": 1234,
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"links": 1,
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"content": [
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"50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d"
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]
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},
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[...]
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]
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}
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This tree contains a file entry. This time, the `subtree` field is not present
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and the `content` field contains a list with one plain text SHA-256 hash.
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The command `restic cat data` can be used to extract and decrypt data given a
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plaintext ID, e.g. for the data mentioned above:
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$ restic -r /tmp/restic-repo cat blob 50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d | sha256sum
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enter password for repository:
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50f77b3b4291e8411a027b9f9b9e64658181cc676ce6ba9958b95f268cb1109d -
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As can be seen from the output of the program `sha256sum`, the hash matches the
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plaintext hash from the map included in the tree above, so the correct data has
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been returned.
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Backups and Deduplication
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=========================
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For creating a backup, restic scans the source directory for all files,
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sub-directories and other entries. The data from each file is split into
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variable length Blobs cut at offsets defined by a sliding window of 64 byte.
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The implementation uses Rabin Fingerprints for implementing this Content
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Defined Chunking (CDC). An irreducible polynomial is selected at random and
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saved in the file `config` when a repository is initialized, so that watermark
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attacks are much harder.
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Files smaller than 512 KiB are not split, Blobs are of 512 KiB to 8 MiB in
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size. The implementation aims for 1 MiB Blob size on average.
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For modified files, only modified Blobs have to be saved in a subsequent
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backup. This even works if bytes are inserted or removed at arbitrary positions
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within the file.
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Threat Model
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============
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The design goals for restic include being able to securely store backups in a
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location that is not completely trusted, e.g. a shared system where others can
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potentially access the files or (in the case of the system administrator) even
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modify or delete them.
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General assumptions:
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* The host system a backup is created on is trusted. This is the most basic
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requirement, and essential for creating trustworthy backups.
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The restic backup program guarantees the following:
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* Accessing the unencrypted content of stored files and meta data should not
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be possible without a password for the repository. Everything except the
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`version` and `id` files and the meta data included for informational
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purposes in the key files is encrypted and authenticated.
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* Modifications (intentional or unintentional) can be detected automatically
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on several layers:
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1. For all accesses of data stored in the repository it is checked whether
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the cryptographic hash of the contents matches the storage ID (the
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file's name). This way, modifications (bad RAM, broken harddisk) can be
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detected easily.
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2. Before decrypting any data, the MAC on the encrypted data is
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checked. If there has been a modification, the MAC check will
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fail. This step happens even before the data is decrypted, so data that
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has been tampered with is not decrypted at all.
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However, the restic backup program is not designed to protect against attackers
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deleting files at the storage location. There is nothing that can be done about
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this. If this needs to be guaranteed, get a secure location without any access
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from third parties. If you assume that attackers have write access to your
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files at the storage location, attackers are able to figure out (e.g. based on
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the timestamps of the stored files) which files belong to what snapshot. When
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only these files are deleted, the particular snapshot vanished and all
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snapshots depending on data that has been added in the snapshot cannot be
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restored completely. Restic is not designed to detect this attack.
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