> For the complete documentation index, see [llms.txt](https://mixfun.gitbook.io/mixfun/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mixfun.gitbook.io/mixfun/concepts/notes-and-custody.md).
# Notes and custody
> How MixFun notes work and what happens if you lose one.
{% hint style="danger" %}
**Lost notes cannot be recovered.** MixFun is non-custodial; there is no operator who can reverse, refund, or re-issue a deposit once the note is gone.
{% endhint %}
A note is the piece of information that proves you deposited into a MixFun pool. Without a note, a deposit cannot be withdrawn. With the note, anyone can withdraw.
MixFun does not store the information that turns a note into a withdrawal. The secret lives only on the device where the note was generated, and on any device you copy it to. This is how privacy and custody work hand in hand: the pool does not know who deposited, and MixFun does not know who can withdraw.
## What a note contains
Every note contains two elements:
* **A secret.** A random value generated in your browser at deposit time. The secret is part of the on-chain commitment. It never leaves your device.
* **A nullifier.** A second random value, derived alongside the secret. The nullifier is revealed at withdrawal time and marked as spent. This prevents the same note from being withdrawn twice.
Both elements are packed into a short text string that you save.
## Why MixFun cannot recover a lost note
If you sign in with your wallet, MixFun stores an encrypted copy of your notes. Encryption happens on your device before the note leaves it, so MixFun only ever sees the ciphertext. Without the decryption key (which lives on your device, alongside your wallet), MixFun cannot read your notes.
If you lose both the note and access to your wallet, the deposit is permanently inaccessible. The funds remain in the pool, waiting for a note that no longer exists. This is a direct tradeoff of non-custodial design: no operator can recover lost funds, and by the same token, no operator can seize them.
## What to do with a note
Treat a note like a private key:
* **Save a copy** somewhere the device can't lose it. Most users screenshot, export, or write the note down.
* **Share only over trusted channels** if the note is for someone else. Anyone with the note can withdraw.
* **Consume quickly** if possible. A note sitting in a draft email or screenshot cache is a lingering risk.
For flow-specific guidance, see [Back up a note](/mixfun/guides/back-up-a-note.md).
## Notes and nullifiers together
A note, once used to withdraw, cannot be reused. The nullifier inside the note is published in the withdrawal proof and recorded on-chain as spent. A second attempt with the same note fails.
This is why a stolen note is only dangerous before its first withdrawal. Once spent, the note is just history.
How pool size affects privacy
Read about anonymity sets to understand how your deposit's privacy scales with the pool.
---
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