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

# Zero-knowledge, without the math

> A plain-language tour of the proofs that make a private balance possible

"Zero-knowledge proof" sounds like something you need a PhD to follow. You don't. This page explains what the proofs do, where they run, and why they matter — with no equations anywhere.

## The one-sentence version

A zero-knowledge proof lets you prove a statement is true without revealing why it is true.

That is the entire idea. You convince a verifier that a claim holds — "this transaction is valid" — while the evidence behind the claim stays with you. The verifier learns exactly one thing: true. Nothing else leaks — not the UTXOs behind the claim, not your balance, not your history, not the reason the claim holds.

## Two ways to picture it

### Over 18, no birthdate

Buying a drink today means handing over an ID card that shows your full birthdate, your address, and a document number — far more than the question required. A zero-knowledge version would confirm one fact: over 18, yes. The bartender learns the single thing they needed and nothing they didn't. Cloak applies the same principle to money: the chain learns "this spend is legitimate" — not which UTXO funded it, what it held, or which wallet deposited it. The link back to you is what stays sealed.

### Sealed envelopes in a public mailbox

Shielded pools are like sealed envelopes in a public mailbox: everyone sees mail, not what's inside. A zero-knowledge proof is what lets you take value back out without opening anything in public. You prove that one of the envelopes is yours and still sealed — without pointing at which one.

## What the proof actually proves in Cloak

When you shield funds, they become UTXOs — the individual private notes inside your private balance. Every time you spend from that balance, your browser produces a proof that convinces the Cloak program of three things:

* **The UTXO you're spending is really yours.** Only someone holding its secret can produce the proof, and the proof never reveals which UTXO in the pool it refers to.
* **It hasn't been spent before.** Each spend publishes a one-time marker called a nullifier. Spend the same UTXO twice and the marker repeats, so the program rejects the transaction — still without learning which UTXO was involved.
* **The amounts balance.** What leaves your UTXOs equals what arrives on the other side — a payment and, usually, change back to you. Nothing created, nothing destroyed. A transaction can never invent value that was never deposited.

Everything else — which UTXO, what it held, the rest of your balance — stays with you.

## Where it happens

In your browser. When you confirm a private send, your device generates the proof locally, typically in under 3 seconds, and attaches it to the transaction. The chain verifies the proof, never the details. Your UTXO secrets are never uploaded, never sent to a server, and never seen by Cloak.

<Tip>
  The proof itself reveals nothing, which is why it is safe to sit on a public chain forever. Even with the proof in hand, no one can work backwards to your UTXOs. That is what "zero-knowledge" means in practice.
</Tip>

## Why this isn't "trust us"

The Cloak program accepts nothing without a valid proof — not from you, not from Cloak's own infrastructure, not from anyone. There is no override, no allowlist, no path where a transaction goes through because the right party asked nicely. The rules are enforced by verification, not reputation. The program itself is upgradeable — how that is governed, and everything else Cloak cannot do, lives on the [Security](/guide/security) page.

That cuts both ways, and honestly so. Cloak can't see your balance, can't spend it, and can't recover it. That's the design. The same proofs that stop a stranger from spending your UTXOs also stop us.

One breadcrumb for the curious: Cloak's proofs are Groth16 proofs, and the pool's data structures use Poseidon hashing. The full construction lives in the [shield pool reference](/protocol/shield-pool).

## Where next

<CardGroup cols={2}>
  <Card title="How Cloak works" icon="route" href="/guide/how-it-works">
    The four flows — shield, send, swap, unshield — and what happens in each.
  </Card>

  <Card title="Your private balance" icon="coins" href="/guide/private-balance">
    UTXOs, the backup file, and what happens if you lose them.
  </Card>
</CardGroup>