> ## 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.

# Zero-knowledge

> How Lastward encrypts your content so only your recipient can read it — never us.

## What we can and can't see

Lastward is honest about its privacy boundary:

| We can see                                       | We can never see                                      |
| ------------------------------------------------ | ----------------------------------------------------- |
| That a switch exists, and its title              | The contents of an encrypted file, secret, or message |
| When it's due and when it fires                  | The passphrase that unlocks it                        |
| The recipient's address (we have to, to deliver) | The decryption key in usable form                     |

The one-line version: **we can see *that* a switch exists and *when* it fires — never *what's
inside*.**

## How the encryption works

Your files and secrets (and, if you choose, your messages) are **zero-knowledge**:

<Steps>
  <Step title="Encrypted on your device">
    When you save an encrypted item, your device encrypts it with a strong modern cipher
    (XChaCha20-Poly1305). The key is derived from a **passphrase you choose** (via Argon2id).
    Only the ciphertext ever leaves your device.
  </Step>

  <Step title="Share the passphrase out-of-band">
    You give the passphrase to your recipient **outside the app** — in a will, a sealed
    envelope, or in person. We never see it, so we can't lose it or hand it over.
  </Step>

  <Step title="Your recipient decrypts in their browser">
    When the switch fires, your recipient opens a link and enters the passphrase. Decryption
    happens **entirely in their browser** — the passphrase is never sent to us.
  </Step>
</Steps>

<Warning>
  Because it's truly zero-knowledge, **if the passphrase is lost, the content cannot be
  recovered** — not even by us. That's the point. Record it somewhere durable (the same place
  you'd keep a will) and make sure your recipient will have it.
</Warning>

## Readable messages

Not everything has to be encrypted. For a plain, low-stakes message you can choose **readable
delivery** — a normal email your recipient just reads, no passphrase needed. It's clearly
labelled as not encrypted when you choose it. Files and secrets are always zero-knowledge.

## Your own account

Your account is always recoverable by email, independently of any switch passphrase. So
forgetting a passphrase never locks you out of *managing* your switches — only out of reading
that one old encrypted payload.