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The Utxo class represents an unspent transaction output (UTXO) in Tornado Nova. A complete transaction consists of 2-16 input UTXOs and 2 output UTXOs.

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Constructor

Initialize a new UTXO with optional parameters. 
const utxo = new Utxo({
  amount: ethers.utils.parseEther('1.0'),
  keypair: myKeypair,
  blinding: randomBN(),
  index: null
})

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Parameters

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amount
BigNumber | BigInt | number | string
default:"0"
The UTXO amount in wei. Can be 0 for dummy UTXOs used to pad transactions.
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keypair
Keypair
default:"new Keypair()"
The keypair that owns this UTXO. If not provided, a random keypair is generated.
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blinding
BigNumber | BigInt | number | string
default:"randomBN()"
A random blinding factor used in commitment calculation. Auto-generated if not provided.
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index
number | null
default:"null"
The UTXO’s position in the Merkle tree. Required for spending the UTXO.

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Methods

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getCommitment()

Returns the commitment for this UTXO. The commitment is calculated using Poseidon hash of the amount, public key, and blinding factor. 
const commitment = utxo.getCommitment()
// Returns: BigNumber
Returns: BigNumber - The commitment value
Commitments are cached after first calculation for performance.

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getNullifier()

Returns the nullifier for this UTXO. The nullifier is used to prevent double-spending. 
const nullifier = utxo.getNullifier()
// Returns: BigNumber
Returns: BigNumber - The nullifier value Throws: Error if the UTXO has amount > 0 but missing index or private key
To compute a nullifier, the UTXO must have:
  • A non-zero amount
  • A valid index in the Merkle tree
  • A keypair with a private key

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encrypt()

Encrypts the UTXO data (amount and blinding factor) using the keypair’s encryption key. 
const encryptedData = utxo.encrypt()
// Returns: '0x...'
Returns: string - Hex string with 0x prefix containing encrypted data The encrypted data includes:
  • Amount (31 bytes)
  • Blinding factor (31 bytes)

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decrypt() (static)

Decrypts UTXO data and creates a new Utxo instance. 
const utxo = Utxo.decrypt(keypair, encryptedData, index)
Parameters:
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keypair
Keypair
required
The keypair used to decrypt the data
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data
string
required
Hex string with encrypted UTXO data
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index
number
required
The UTXO’s index in the Merkle tree
Returns: Utxo - A new Utxo instance with decrypted data

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Example usage

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Creating a deposit UTXO

const { Utxo, Keypair } = require('./src/index')
const { ethers } = require('hardhat')

// Create keypair
const keypair = new Keypair()

// Create UTXO for 1 ETH deposit
const depositUtxo = new Utxo({
  amount: ethers.utils.parseEther('1.0'),
  keypair: keypair
})

console.log('Commitment:', depositUtxo.getCommitment().toString())

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Creating dummy UTXOs

Transactions require exactly 2 or 16 inputs and 2 outputs. Use dummy UTXOs with amount 0 to pad: 
// Create dummy UTXO (amount defaults to 0)
const dummyUtxo = new Utxo()

// Pad outputs array
const outputs = [realUtxo, new Utxo()]

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Decrypting received UTXOs

// Get encrypted output from transaction event
const encryptedOutput = event.args.encryptedOutput1

// Decrypt with your keypair
const receivedUtxo = Utxo.decrypt(
  myKeypair,
  encryptedOutput,
  utxoIndex
)

console.log('Received amount:', receivedUtxo.amount.toString())

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Spending a UTXO

// UTXO must have index set to be spent
inputUtxo.index = 5 // Set from Merkle tree

try {
  const nullifier = inputUtxo.getNullifier()
  console.log('Nullifier:', nullifier.toString())
} catch (error) {
  console.error('Cannot spend UTXO:', error.message)
}

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Properties

After construction, a Utxo instance has these properties:
  • amount - BigNumber representing the UTXO amount
  • blinding - BigNumber representing the blinding factor
  • keypair - The Keypair that owns this UTXO
  • index - Number or null, the Merkle tree index

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