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

# How Atrum works

> The whole mechanism in one page, before the details.

Atrum is a parimutuel prediction market in which the size of every position is encrypted. This
page is the complete picture; the pages after it go deeper on each part.

## The core idea

Three things have to be true at once, and they pull against each other:

1. The pool needs a **total**, or winnings cannot be divided.
2. Individual stakes must stay **hidden**, or the market punishes anyone with an edge.
3. Everything must be **verifiable**, because it is a public chain and nobody should have to
   trust an operator.

Atrum gets all three by never letting the contract see a stake, while still letting it add
them up.

```mermaid theme={null}
flowchart TD
  subgraph hidden["Hidden"]
    N["your note<br/>value, secrets"]
    ST["your stake"]
  end

  subgraph pub["Public and verifiable"]
    C["commitment in the tree"]
    NH["nullifier hash"]
    CT["ciphertext"]
    TOT["encrypted pool total"]
  end

  N -->|"Poseidon hash"| C
  N -->|"one-way"| NH
  ST -->|"ElGamal"| CT
  CT -->|"point addition"| TOT
  TOT -->|"proved decryption<br/>at settlement only"| PAY["payouts"]

  style hidden stroke:#15803d,stroke-width:2px
  style pub stroke:#b91c1c,stroke-width:2px
```

## The four moves

**Deposit.** Lock collateral, publish a commitment to a note only you can spend. The amount is
public and must be a standard denomination, so it does not single you out.

**Bet.** Prove you own *some* unspent note without revealing which, and publish an encrypted
stake. The contract adds the ciphertext to a running total it cannot read. This is where the
link from your address to your position is cut.

**Redeem.** Once the market settles, prove your position won and receive a new note holding the
payout. **No money moves.** Nothing about you is published. This is where the link from your
position to your money is cut.

**Withdraw.** Turn a settled note into collateral in your wallet, at a time you choose, in a
standard amount. Public — but unconnected to anything above.

## Why two separate exits

Redeeming and withdrawing look like they could be one step. Combining them would undo the
product.

A parimutuel payout is whatever the arithmetic produced — 1,041, an odd number determined by
your stake and the pool. Paying that directly to an address publishes an amount that
**uniquely identifies the position that earned it**. Anyone could work backwards from the
payout to the bet.

So redemption keeps the payout inside the system as a note, and withdrawal takes out a *round*
amount later, keeping the remainder as private change. Two steps, because one would leak.

## What the contract never learns

* Which note any action spent
* How much any bet was for
* The pool total, at any point before settlement
* Which deposit funded which position
* Which position funded which withdrawal

## What is public, deliberately

* That a bet happened, and **which side** it backed
* Deposit and withdrawal amounts, in standard denominations
* The final pool totals, **after** betting closes and the outcome is known
* Coarse mid-market odds while betting is open

The full accounting is in [Privacy model](/concepts/privacy-model).

## Where to go next

<CardGroup cols={2}>
  <Card title="Notes and commitments" icon="key" href="/concepts/notes-and-commitments">
    How ownership works without a record of who owns what.
  </Card>

  <Card title="Encrypted totals" icon="calculator" href="/concepts/encrypted-totals">
    How a contract adds numbers it is not allowed to read.
  </Card>

  <Card title="Why parimutuel" icon="scale-balanced" href="/concepts/why-parimutuel">
    The constraint that decides the entire market design.
  </Card>

  <Card title="Resolution" icon="gavel" href="/concepts/resolution">
    Why the outcome is a computation and not a decision.
  </Card>
</CardGroup>