replicat 1.2.3

Configurable and lightweight backup utility with deduplication and encryption.

[ ˈrɛplɪkət ]

Replicat

Configurable and lightweight backup utility with deduplication and encryption.

Compatibility

Python 3.8 (or newer) running on Linux, MacOS, or Windows.

Supported backup destinations

• local path
• Backblaze B2
• Amazon S3
• any S3-compatible service

You can implement and use your own adapter for pretty much any backup destination without changing the source code of Replicat (more on that later).

Installation

It's available on PyPI, so

pip install replicat

Reasoning

For various reasons, I wasn't 100% happy with any of the similar projects that I've tried. It's likely that I will never be 100% happy with Replicat either, but at least it will be easier for me to fix problems or add new features.

Highlights/goals:

• efficient, concise, easily auditable implementation
• high customizability
• few external dependencies
• well-documented behaviour
• unified repository layout
• API that exists

This project borrows a few ideas from those other projects, but not enough to be considered a copycat.

Introduction

You can use Replicat to backup files from your machine to a repository, located on a backend such as a local directory or cloud storage (like Backblaze B2). Your files are transfered and stored in an optionally encrypted and chunked form, and references to chunks (i.e. their hash digests) are stored in snapshots along with file name and metadata.

Replicat supports two types of repositories: encrypted (the default) and unencrypted.

Chunks, snapshots, and all other pieces of data inside unencrypted repositories are stored unencrypted. The storage names for chunks and snapshots are simply the hash digests of their contents.

Currently, the only supported type of encryption is symmetric encryption. To use symmetric encryption, you will need a key and the password associated with that key. A key contains parameters for the KDF and an encrypted (private) section, which can only be decrypted by the owner of the key using the matching password. That section contains secrets for the cryptographic primitives that control how the data is split into chunks, visibility of chunks of data, and more.

You can create multiple keys with different passwords and settings. When adding a new key to a repository with symmetric encryption, you'll have to unlock it with one of the existing keys. You have a choice to either share secrets with the other key OR generate new secrets. Owners of keys with shared secrets ("shared" keys) can use deduplication features together, i.e., chunks of data that were uploaded by the owner of one such key can be accessed and decrypted by the owner of the other key. Assume that they will also be able to check whether you have a specific piece of data. To avoid such risk, you can create a key with new secrets (an "independent" key). That way, Replicat will isolate your data and make it inaccessible to the owners of other keys. Of course, if you use your key to create a yet another (new) key, you will also have the ability to share your secrets with others, even if they were originally copied from some other key. This creates a web of trust of sorts.

In contrast with unencrypted repositories, the storage name for the chunk is derived from the hash digest of its contents and one of the aforementioned secrets, in order to reduce the chance of successful "confirmation of file" attacks. The chunk itself is encrypted with the combination of the hash digest of its contents and another one of those secrets, since the usual convergent encryption is vulnerable to that same "confirmation of file" attack. Table of chunk references inside a snapshot is encrypted similarly, but the list of files that reference those chunks is encrypted using the key and the password that were used to unlock the repository, and therefore can only be decrypted by the owner of that key (even in the case of shared secrets). A snapshot created using an independent key will not be visible. A snapshot created using a shared key will be visible, but there will be no available information about it beyond its storage name and the table of chunk references.

Deeper dive

You're about to see diagrams illustrating how replicat processes data, along with example contents of the configuration file, keys, and snapshots. Here's the terminology:

• Encrypt(data, key)/Decrypt(data, key) -- encrypts/decrypts data with the encryption key key using an authenticated encryption algorithm. It's normally used to encrypt/decrypt private sections in keys, as well as chunks and snapshots.

• Hash(data) -- computes the hash digest of data using a hashing algorithm. It's used to check integrity of data and to derive encryption keys for chunks and snapshots.

• Mac(data, key) -- computes the message authentication code for data using suitable key and a MAC algorithm. It's mainly used to verify ownership of chunks.

• SlowKdf(ikm, salt[, context])/FastKdf(ikm, salt[, context]) -- calls a "slow"/"fast" key derivation function to obtain an encryption key from ikm using salt and an optional context. As a general rule, replicat uses "slow" KDF for low-entropy inputs and "fast" KDF for high-entropy inputs. The output length will match the encryption key length of the chosen encryption algorithm.

• UserKey -- encryption key derived as SlowKdf(Password, UserKdfParams), where Password is the user's password and UserKdfParams is the salt. UserKey is used to encrypt sensitive personal data: private sections in keys and file metadata in snapshots.

• SharedKey, SharedKdfParams, SharedMacKey, SharedChunkerKey -- secrets stored in the private sections of keys. SharedKey and SharedKdfParams are used to derive encryption keys using "fast" KDF (they will encrypt shared data, like chunks and chunk references). SharedMacKey is the MAC key. SharedChunkerKey personalises content-defined chunking (CDC) to prevent watermarking attacks.

• GetChunkLocation(name, authentication_tag)/GetSnapshotLocation(name, authentication_tag) -- obtains the location for a chunk/snapshot using its name and the corresponding authentication tag.

• Upload(data, location) -- uploads data to the backend to the given location.

• Download(location) -- downloads data from the backend at the given location.

Command line interface

The installer will create the replicat command (same as python -m replicat). There are several available subcommands:

• init -- initializes the repository using the provided settings
• snapshot -- creates a new snapshot in the repository
• list-snapshots/ls -- lists snapshots
• list-files/lf -- lists files across snapshots
• restore -- restores files from snapshots
• add-key -- creates a new key for the encrypted repository
• delete -- deletes snapshots by their names
• clean -- performs garbage collection
• upload -- uploads files to the backend (no chunking, no encryption, keeping original names)

⚠️ WARNING: actions that read from or upload to the repository can safely be run concurrently; however, there are presently no guards in place that would make it safe for you to run destructive actions (delete, clean) concurrently with those actions unless you use independent keys (see the explanation above). I do plan to implement them soon-ish, but in the meantime DO NOT use shared keys (or, naturally, the same key) to snapshot and clean at the same time, for example.

As far as the upcoming implementation of such guards, it'll be based on locks. I'm familiar with the lock-free deduplication strategy (like in Duplicacy), but I don't like it much.

There are several command line arguments that are common to all subcommands:

• -r/--repository -- used to specify the type and location of the repository backend (backup destination). The format is <backend>:<connection string>, where <backend> is the short name of a Replicat-compatible backend and <connection string> is open to interpretation by the adapter for the selected backend. Examples: b2:bucket-name for the B2 backend or local:some/local/path for the local backend (or just some/local/path, since the <backend>: part can be omitted for local destinations). If the backend requires additional arguments, they will appear in the --help output. Refer to the section on backends for more detailed information.

• -q/--hide-progress -- suppresses progress indication for commands that support it

• -c/--concurrent -- the number of concurrent connections to the backend

• --cache-directory -- specifies the directory to use for cache. --no-cache disables cache completely.

• -v/--verbose -- specifies the logging verbosity. The default verbosity is WARNING, -v means INFO, -vv means DEBUG.

Encrypted repositories require a key and a matching password for every operation:

• -K/--key-file -- the path to the key file
• -p/--password -- the password in plaintext. However, it's more secure to provide the password in a file via the -P/--password-file argument, or as an environment variable REPLICAT_PASSWORD.

init examples

# Unencrypted repository in some/directory. The --encryption none flag disables encryption
$ replicat init -r some/directory --encryption none
# Encrypted repository with initial password taken from string.
# The new key will be printed to stdout
$ replicat init -r some/directory -p 'password string'
# Encrypted repository with initial password taken from a file.
# The new key will be written to path/to/key/file
$ replicat init -r some/directory -P path/to/password/file -o path/to/key/file
# Specifies the cipher
$ replicat init -r some/directory -p '...' --encryption.cipher.name chacha20_poly1305
# Specifies the cipher name and parameters
$ replicat init -r some/directory \
    -p '...' \
    --encryption.cipher.name aes_gcm \
    --encryption.cipher.key_bits 128
# Specifies the KDF name and parameters (for the key)
$ replicat init -r some/directory \
    -p '...' \
    --encryption.kdf.name scrypt \
    --encryption.kdf.n 1048576
# Specifies the chunking parameters
$ replicat init -r some/directory \
    -p '...' \
    --chunking.min-length 128_000 \
    --chunking.max-length 2_048_000
# Equivalent (dashes in argument names are converted to underscores)
$ replicat init -r some/directory \
    -p '...' \
    --chunking.min_length 128_000 \
    --chunking.max_length 2_048_000

snapshot examples

# Unlocks the repository, uploads provided files in encrypted chunks,
# using no more than 10 concurrent connections, creating a snapshot
$ replicat snapshot -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    -c 10 \
    -n 'A note (optional)'
    image.jpg some-directory another-directory and/more.text

list-snapshots/ls examples

# Unlocks the repository and lists all of the snapshots
$ replicat list-snapshots -r some/directory -P path/to/password/file -K path/to/key/file
# Equivalent
$ replicat ls -r some/directory -P path/to/password/file -K path/to/key/file

list-files/lf examples

# Unlocks the repository and lists all versions of all the files
$ replicat list-files -r some/directory -P path/to/password/file -K path/to/key/file
# Equivalent
$ replicat lf -r some/directory -P path/to/password/file -K path/to/key/file
# Only lists files with paths matching the -F regex
$ replicat lf -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    -F '\.(jpg|text)$'

restore examples

# Unlocks the repository and restores the latest versions of all files to target-directory
$ replicat restore -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    target-directory
# Unlocks the repository and restores the latest versions of files with paths matching the
# -F regex in snapshots matching the -S regex to 'target-directory'
$ replicat restore -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    -F '\.(jpg|text)$' \
    -S 'abcdef' \
    target-directory

add-key examples

# Unlocks the repository and creates an independent key, which will be printed to stdout
$ replicat add-key -r some/directory -P path/to/password/file -K path/to/key/file
# Unlocks the repository and creates a shared key (i.e. with shared secrets)
$ replicat add-key -r some/directory -P path/to/password/file -K path/to/key/file --shared
# Unlocks the repository and creates an independent key, which will be written
# to path/to/new/key/file
$ replicat add-key -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    -o path/to/new/key/file
# Unlocks the repository and creates an independent key with some custom settings
# (cipher params as well as chunking and hashing settings are repository-wide)
$ replicat add-key -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    --encryption.kdf.name scrypt \
    --encryption.kdf.n 1048576

delete examples

# Unlocks the repository and deletes snapshots by name (as returned by ls/list-snapshots).
# Chunks that aren't referenced by any other snapshot will be deleted automatically
$ replicat delete -r some/directory \
    -P path/to/password/file \
    -K path/to/key/file \
    NAME1 NAME2 NAME3 ...

clean examples

# Unlocks the repository and deletes all chunks that are not referenced by any snapshot
$ replicat clean -r some/directory -P path/to/password/file -K path/to/key/file

upload examples

# Uploads files directly to the backend without any additional processing.
# File path -> resulting name:
#   /working/directory/some/file -> some/file
#   /working/directory/another/file -> another/file
#   /working/directory/another/directory/another-file -> another/directory/another-file
#   /absolute/directory/path/with-file -> absolute/directory/path/with-file
#   /absolute/file -> absolute/file
/working/directory$ replicat upload -r some:repository \
                        some/file \
                        /working/directory/another/directory \
                        /absolute/directory/path \
                        /absolute/file
# Uploads files that do not yet exist in the repository (only checks the file names)
$ replicat upload -r some:repository --skip-existing some/file some/directory

Check version

replicat --version

Backends

Run replicat commands with -r <backend>:<connection string> and additional arguments that are specific to the selected backend. Those arguments may have defaults and may also be provided via environment variables. Use

replicat <command> -r <backend>:<connection string> --help

to see them.

Local

The format is -r local:some/local/path or simply -r some/local/path.

B2

The format is -r b2:bucket-id or -r b2:bucket-name. This backend uses B2 native API. The required arguments are --key-id (keyId) and --application-key (applicationKey). Sign into your Backblaze B2 account to generate them. You can use master application key or a normal application key (which can also be restricted to a single bucket).

S3

The format is -r s3:bucket-name. Requires arguments --key-id, --access-key, and --region.

S3-compatible

The format is -r s3c:bucket-name. Requires arguments --key-id, --access-key, --region, and --host. --host must not include the scheme. The default scheme is https, but can be changed via the --scheme argument.

Custom backends

replicat.backends is a namespace package, making it possible to add custom backends without changing replicat source code.

Suppose your backend of choice is a hypothetical low low cost cloud storage Proud Cloud (pc for short). The most barebones implementation of the Replicat-compatible adapter for the pc backend will require a directory with the following structure:

$ tree proud-cloud/
proud-cloud/
└── replicat
    └── backends
        └── pc.py

The -r argument of replicat commands will take the form of -r pc:<connection string>. Replicat will use it to locate the pc module inside the replicat.backends package, load the replicat.backends.pc.Client class, and pass the <connection string> to its constructor to create the backend instance. In case there are some additional parameters that are required to connect to Proud Cloud (account id, secret token, etc.), you should add them to the replicat.backends.pc.Client constructor as keyword-only arguments. If present, Replicat will generate the corresponding command line arguments with defaults and you'll even be able to use environment variables to provide them.

replicat.backends.pc.Client must subclass replicat.backends.base.Backend and implement all of the methods marked as abstract. You could use implementations of existing replicat.backends.base.Backend subclasses as examples. To make your implementation visible to Replicat, you'll need to add it to the module search path before invoking replicat (you could use the PYTHONPATH environment variable for that).

Here's an example:

# ./proud-cloud/replicat/backends/pc.py
from .base import Backend

class ProudCloud(Backend):
    def __init__(self, connection_string, *, account_id, secret, port=9_876, legacy=False):
        print(f'PC args: {connection_string=}, {account_id=}, {secret=}, {port=}, {legacy=}')
        ...
    ...

Client = ProudCloud

$ PYTHONPATH=proud-cloud replicat init -r pc:... --help
usage: replicat init [-h] [-r REPO] [-q] [-c CONCURRENT] [-v] [-K KEYFILE]
                     [-p PASSWORD | -P PASSWORD_FILE_PATH] [--account-id ACCOUNT_ID]
                     [--secret SECRET] [--port PORT] [--legacy LEGACY] [-o KEY_OUTPUT_FILE]

optional arguments:
  ...

arguments specific to the ProudCloud backend:
  --account-id ACCOUNT_ID
                        or the PROUDCLOUD_ACCOUNT_ID environment variable
  --secret SECRET       or the PROUDCLOUD_SECRET environment variable
  --port PORT           or the PROUDCLOUD_PORT environment variable, or the constructor default 9876
  --legacy LEGACY       or the PROUDCLOUD_LEGACY environment variable, or the constructor default False

$ PYTHONPATH=proud-cloud PROUDCLOUD_LEGACY=true PROUDCLOUD_SECRET='pr0ud' \
    replicat init -r pc:... \
    --account-id 12345 \
    --port 9877
PC args: connection_string='...', account_id=12345, secret='pr0ud', port=9877, legacy=True
...

If you've created a Replicat-compatible adapter for a backend that Replicat doesn't already support and your implementation doesn't depend on additional third-party libraries (or at least they are not too heavy and can be moved to extras), consider submitting a PR to include it in this repository.

Security

If you believe you've found a security issue with Replicat, please report it to [e-mail] (or DM me on Twitter or Telegram).