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Describe threat model

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Alexander Neumann 2015-01-03 15:28:33 +01:00
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@ -269,3 +269,46 @@ The implementation aims for 1MiB chunk size on average.
For modified files, only modified chunks 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 content of backup-up files and meta data should not be possible
without a password for the repository. Everything except the `version` file
and the meta data included for informational purposes in the key files is
encrypted and then signed.
* 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 HMAC signature on the encrypted data is
checked. If there has been a modification, the signature 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.