> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/Privacy-Cash/privacy-cash/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction to Privacy Cash

> Transfer SOL and SPL tokens privately on Solana using zero-knowledge proofs

<img className="block dark:hidden" src="https://mintlify.s3.us-west-1.amazonaws.com/privacy-cash-privacy-cash/images/hero-light.svg" alt="Privacy Cash Hero Light" />

<img className="hidden dark:block" src="https://mintlify.s3.us-west-1.amazonaws.com/privacy-cash-privacy-cash/images/hero-dark.svg" alt="Privacy Cash Hero Dark" />

## Overview

Privacy Cash is a privacy protocol on Solana that enables confidential transactions using zero-knowledge proofs. The protocol allows users to shield and withdraw SOL (with SPL token support coming soon) without revealing transaction linkages.

The program is fully audited by Accretion, HashCloak, Zigtur and Kriko, and verified onchain with hash:

```
c6f1e5336f2068dc1c1e1c64e92e3d8495b8df79f78011e2620af60aa43090c5
```

<CardGroup cols={2}>
  <Card title="Quick start" icon="rocket" href="/quickstart">
    Get started with Privacy Cash in minutes
  </Card>

  <Card title="Installation" icon="download" href="/installation">
    Install dependencies and set up your environment
  </Card>

  <Card title="Privacy Cash SDK" icon="code" href="https://github.com/Privacy-Cash/privacy-cash-sdk">
    Integrate Privacy Cash into your project
  </Card>

  <Card title="GitHub Repository" icon="github" href="https://github.com/Privacy-Cash">
    View the source code and contribute
  </Card>
</CardGroup>

## How it works

Privacy Cash implements a privacy protocol with two core operations:

<Steps>
  <Step title="Shield SOL">
    Deposit SOL into a privacy pool, generating a commitment that is added to a Merkle tree. The commitment contains your amount and a secret keypair, but reveals nothing about the owner.
  </Step>

  <Step title="Withdraw SOL">
    Withdraw SOL from the privacy pool to any recipient address using zero-knowledge proofs. The proof demonstrates you own a valid commitment in the tree without revealing which one.
  </Step>
</Steps>

<Note>
  The implementation uses zero-knowledge proofs to ensure that withdrawals cannot be linked to deposits, providing privacy for Solana transactions.
</Note>

## Key features

### Zero-knowledge privacy

Privacy Cash uses Groth16 zero-knowledge proofs on the BN254 elliptic curve to verify transactions without revealing sensitive information. Each transaction proves:

* The spender owns valid commitments in the Merkle tree
* The input and output amounts balance correctly
* Nullifiers have not been used before (preventing double spends)

All without revealing which commitments are being spent or linking deposits to withdrawals.

### Merkle tree commitment scheme

Commitments are stored in a sparse Merkle tree with:

* **Height**: 26 levels
* **Root history**: 100 previous roots maintained for proof flexibility
* **Poseidon hash**: Gas-efficient hash function optimized for zero-knowledge circuits

Each commitment is computed as:

```typescript theme={null}
commitment = PoseidonHash(amount, publicKey, blinding, mintAddress)
```

### UTXO model

Privacy Cash implements a UTXO (Unspent Transaction Output) model similar to Bitcoin and Tornado Cash Nova:

<CodeGroup>
  ```typescript UTXO structure theme={null}
  export class Utxo {
    amount: BN;
    blinding: BN;           // Random value for hiding commitment
    keypair: Keypair;       // Private key signs, public key = PoseidonHash(privateKey)
    index: number;          // Position in Merkle tree
    mintAddress: string;    // SOL or SPL token mint

    async getCommitment(): Promise<string> {
      const mintAddressField = getMintAddressField(new PublicKey(this.mintAddress));
      return this.lightWasm.poseidonHashString([
        this.amount.toString(), 
        this.keypair.pubkey.toString(), 
        this.blinding.toString(), 
        mintAddressField
      ]);
    }

    async getNullifier(): Promise<string> {
      const commitment = await this.getCommitment();
      const signature = this.keypair.sign(commitment, this.index.toString());
      return this.lightWasm.poseidonHashString([
        commitment, 
        this.index.toString(), 
        signature
      ]);
    }
  }
  ```
</CodeGroup>

### Fee structure

<CardGroup cols={2}>
  <Card title="Deposit fee" icon="arrow-down">
    **0%** - Deposits are completely free
  </Card>

  <Card title="Withdrawal fee" icon="arrow-up">
    **0.25%** - Small fee on withdrawals (25 basis points)
  </Card>
</CardGroup>

<Info>
  Fees are configurable by the protocol authority and include a 5% error margin for tolerance.
</Info>

### Supported tokens

**Mainnet**: USDC, USDT, ORE, ZEC, stORE, jlUSDC, jlWSOL\
**Devnet**: USDC, USDT, ORE, ZEC, stORE, jlUSDC, jlWSOL (testnet versions)

<Note>
  Additional SPL tokens can be enabled by the protocol authority through the `initialize_tree_account_for_spl_token` instruction.
</Note>

## Program addresses

<CodeGroup>
  ```bash Mainnet theme={null}
  9fhQBbumKEFuXtMBDw8AaQyAjCorLGJQiS3skWZdQyQD
  ```

  ```bash Devnet theme={null}
  ATZj4jZ4FFzkvAcvk27DW9GRkgSbFnHo49fKKPQXU7VS
  ```
</CodeGroup>

## Security

Privacy Cash has been audited by multiple security firms:

* **Accretion** - Smart contract security audit
* **HashCloak** - Zero-knowledge proof audit
* **Zigtur** - Cryptography review
* **Kriko** - Full protocol audit

The program is verifiable onchain, allowing anyone to verify the deployed bytecode matches the audited source code.

<Warning>
  While the protocol has been audited, use at your own risk. Never deposit more than you can afford to lose.
</Warning>

## Architecture

The Privacy Cash repository is structured as follows:

```
privacy-cash/
├── anchor/              # Solana program (smart contract)
│   ├── programs/
│   │   └── zkcash/
│   │       └── src/     # Rust source code
│   └── tests/           # TypeScript integration tests
├── circuits/            # Zero-knowledge circuit definitions
├── artifacts/           # Compiled circuit artifacts
│   └── circuits/
│       ├── transaction2_js/
│       └── verifyingkey2.json
└── scripts/             # Deployment and utility scripts
```

## Next steps

<CardGroup cols={2}>
  <Card title="Quick start guide" icon="play" href="/quickstart">
    Start using Privacy Cash with our quick start tutorial
  </Card>

  <Card title="Install dependencies" icon="terminal" href="/installation">
    Set up your development environment
  </Card>
</CardGroup>