💡 A Paymaster defines the gas abstraction logic for a smart contract wallet.

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What is Gas Abstraction?

Gas abstraction enables:
  • Gas payment in alternative ERC20 tokens
  • Gasless transactions (requiring gas sponsorship from a Paymaster)

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Key Features of Paymasters

  1. Modular Design: ERC-4337 allows a UserOperation to choose any Paymaster by specifying its address and input parameters in the paymasterAndData field.
  2. Smart Contract Implementation: A Paymaster is essentially a smart contract implemented by a dApp. It uses custom logic in the validatePaymasterOp function, which the Bundler calls to selectively pay gas for UserOperations.
  3. ETH Deposits: Paymasters must deposit Ethereum on the Entry Point Contract to pay for gas for UserOperations.
  4. Staking Requirement: In addition to gas deposits, Paymasters must stake ETH on the Entry Point Contract. This stake acts as an anti-spam measure rather than being slashed.

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Paymaster Interface

The Paymaster interface exposes two crucial functions:

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1. validatePaymasterOp

  • Defines the logic for gas abstraction
  • Ensures sufficient balance (e.g., checking if the wallet has enough ERC20 tokens for gas payment)
  • Called before the execution of a UserOperation
  • Follows the principle of separating validation from execution

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2. postOp

  • Called at the final step of a smart contract wallet function call
  • Can revert the function call if the wallet lacks sufficient ERC20 balance to reimburse the Paymaster
  • Double-checks gas sufficiency after function execution
  • Gas is still deducted even if a function call fails at this step