Fees

Nullmask uses gas abstraction — the relayer pays gas fees on behalf of the user, and is reimbursed from the shielded transaction's fee allocation.

How Fees Work

1. The proxy estimates the fee before proof generation using nullmask_estimateFee

2. The fee is deducted from the user's shielded balance

3. The ZK proof includes the fee amount and fee token as public inputs

4. The contract transfers the fee to the relayer (msg.sender) after verifying the proof

Fee Estimation

fee = gasEstimate × currentGasPrice × exchangeRate

• Gas estimate: Pre-computed per action type (transfer, withdrawal, swap)

• Gas price: Fetched from the blockchain at estimation time

• Exchange rate: Conversion rate if paying in a non-ETH token

Fee Tokens

Fees can be paid in:

• ETH (native token)

• Same token as the shielded action

When action asset and fee asset are the same:

• Input notes contribute to a combined pool

• A single change note is created

When they differ:

• Separate note pools for action and fee values

• Two change notes are created

Fee by Action Type

Action	Default Gas Estimate
Shielded Transfer	4,000,000 gas
Shielded Withdrawal	4,000,000 gas
Shielded Swap	5,000,000 gas
Approve Deposit	500,000 gas

A 1.25x buffer is applied on top of these defaults. The service also tracks actual gas usage from recent transactions and uses historical data when available.

No Gas in Wallet

Users do not need ETH in their wallet to perform shielded operations. The relayer pays all on-chain gas costs. Users only need ETH (or another token) in their shielded balance to cover fees.

The only operations requiring wallet ETH are:

• Initial deposit to the privacy pool

• On-chain key registration (during onboarding)