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How your data is protected

These are the promises Deucalion makes about your data, and how they were checked.

The promises

A backup only reads your computer. It never changes, moves or deletes anything on your computer. Tests compare every file's content, permissions and change time before and after each backup.

Nothing on the drive is deleted by a backup. Changed and deleted files go to Removed Files. A test checks that every version ever backed up is still on the drive afterwards, either in the backup or in Removed Files.

Backups go only to a drive you registered. A backup drive is recognised by an identifier written on it, not by its name or where it is mounted. Deucalion refuses to back up to the disk your system runs from: "… is on the same disk as your system. A backup there would be lost together with the system. Choose an external drive."

A restore never loses a file on your computer. Anything a restore replaces is first renamed into the undo folder ~/Deucalion-before-restore/ (never copied and then deleted). If a restore fails or is stopped, the original is put back. Folders where another drive is mounted are never touched. Every restore shows a preview first.

A rebuild keeps what it replaces. Desktop settings, crontabs and /etc files are saved first. Packages are never removed. Ubuntu's own software sources are never changed. Paths in settings are updated only as whole paths (/home/alex/… is updated, /home/alexandra is not). Running a rebuild twice changes nothing more.

Interruptions are safe. You can stop a backup, kill it, unplug the drive or let the drive run full at any moment: the next backup repairs the backup, with no duplicates and no damage. See FAQ: power cut or unplugged drive.

Erasing a drive needs you to type its name. Preparing a drive in a way that erases it asks you to type its device name (such as sdb). Your system disk is never offered, and the administrator helper checks this again on its own.

Edits that keep the same size and date are still found. Some programs change a file and put its modification date back. Deucalion also checks the file's change time, which programs can't set, and compares the content when in doubt.

VeraCrypt is installed only after its signature is checked against VeraCrypt's own key (see How VeraCrypt is installed).

Your backup password is never stored. Deucalion passes it straight to the encryption software and forgets it.

How this was tested

  • Unit and integration tests (tests/): on plain folders, real exFAT and LUKS volumes, including regression tests for every problem found in three independent code reviews and while writing this documentation.
  • A randomized torture test (tests/torture.py): random folders full of awkward names, and backups and restores killed at random moments. Hundreds of runs passed on plain, exFAT and LUKS drives.
  • Administrator checks (tests/manual/admin_checks.py, veracrypt_checks.py): backups with protected settings, with /etc fingerprinted before and after; an attack on the administrator helper with symbolic links, and refused malicious requests; VeraCrypt create, open, wrong password and close in all three encrypted layouts.
  • A full rebuild of a fresh Ubuntu 26.04 virtual machine from a 24.04 backup, under a different user name, run twice (tests/manual/vm_rebuild_check.py).
  • A real-data backup of about 700,000 files (49 GB): verified, restored and compared byte for byte.
  • This documentation: every tutorial was followed step by step, in the app and in the terminal, on throwaway Ubuntu 24.04 and 26.04 virtual machines with demo drives. The screenshots and the terminal output on this site come from those runs.

See Tests for how to run them.

What Deucalion can't protect you from

  • A forgotten password. An encrypted backup can't be opened without its password. Nobody can recover it.
  • One drive is one point of failure. If your only backup drive breaks, is lost or is stolen together with your computer, the backup is gone. Keep the drive somewhere else when you don't use it, and consider a second drive.
  • Files you never backed up. Deucalion runs only when you start it.