> ## Documentation Index
> Fetch the complete documentation index at: https://docs.burakov.net/llms.txt
> Use this file to discover all available pages before exploring further.

# Sync & encryption

> How Tidybook keeps one address book identical across your devices, why the server stays blind, and the difference between the default and the zero-knowledge passphrase lock.

Sync is the point of Tidybook: one tidy book, everywhere, private. It's part of the **Sync** plan.

## Multi-device, offline-first

Your book lives on each device, so every screen is instant and works with no connection. When you're
online, Tidybook reconciles your devices in the background.

Edits **fold together at the level of individual fields**, so two devices editing the same person
don't fight — the changes combine. Tidybook is deliberately cautious about removals: if one device
adds a phone number while another removes it, it **keeps** the number. With contacts, quietly losing
a number is the worst outcome, so ties go to keeping your data. You'll never see a "which version
wins?" dialog.

## The server is blind

Encryption happens on your device **before anything leaves it**. What reaches the server is
**ciphertext** — a sealed blob it cannot open. It relays that blob to your other devices and never
sees a name, number or note. Running the service, I only ever see encrypted data.

## Default mode vs. the passphrase lock

Tidybook offers two honest levels of protection:

* **Default (convenient).** Tidybook manages your encryption key so you can recover your book simply
  by signing in on a new device. Your data is encrypted in transit and at rest — but because the key
  is recoverable through your account, this is **not** zero-knowledge.
* **Passphrase lock (zero-knowledge).** Set a passphrase and your key is derived from something only
  you know. After that, **not even Tidybook can decrypt your contacts**. New devices need the
  passphrase (or your one-time recovery code).

## Recovery — read this before locking

The trade with the passphrase lock is real and stated plainly: if you lose both the passphrase and
the recovery code, **no one can recover your book** — that's what zero-knowledge means. Always keep a
[vCard / CSV export](/tidybook/cleanup-and-editing) as an independent cold backup before you enable
the lock.

<Note>
  Whichever mode you choose, a vCard / CSV export is your key-independent safety net. Make one before
  changing your encryption settings.
</Note>