Design Patterns

Practical design approaches for safe, scalable, and efficient Paymasters.

Beyond basic logic, well-architected Paymasters benefit from patterns that improve security, flexibility, and user experience. This section outlines common design techniques used in production-ready Paymasters.

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🧰 Whitelist with Expiry

Grant gas sponsorship based on a short-lived whitelist:

• Store (address => expiry timestamp) mapping

• Only allow validatePaymasterUserOp() if now < expiry

• Good for beta access or trial runs

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🔐 Signed API Approvals

The off-chain component signs metadata that gets embedded in paymasterAndData:

• Signature includes sender, callData hash, deadline, quota

• Contract validates signature on-chain

• Prevents reuse or forgery; allows detailed policies

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⏳ Rate Limits and Quotas

Track per-user limits:

• Tx count per hour

• Gas budget per week

• Use a rolling window or resettable counter

Store these in ephemeral in-memory maps or external storage if staked.

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💸 ERC-20 Gas Payment with Allowance

Let users pre-approve ERC-20 tokens:

• Call transferFrom() in postOp()

• Pull tokens only after successful execution

• Ensure price quotes are stable and front-run resistant

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📉 Fallback and Failover Logic

If a Paymaster becomes invalid or fails:

• Fallback to an unauthenticated Paymaster for emergency gas

• Use context returned by validatePaymasterUserOp() to pass failover hints

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🔄 Modular Composition

Split Paymaster logic into modules:

• Ruleset manager (e.g. quotas, blocklists)

• Signature verifier

• Gas accounting engine

This makes audits easier and allows permissionless extensions.

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📘 ERC-7677 Standard

ERC-7677 standardizes the Paymaster interface and lifecycle for ERC-4337 systems. It defines:

• Minimal required methods

• Paymaster staking rules

• Shared validation behaviors

This makes it easier for bundlers and wallets to interoperate with any compliant Paymaster contract. If you're building a general-purpose or pluggable Paymaster, following ERC-7677 is highly recommended.

✅ Summary

Robust Paymasters combine simplicity with modular safeguards. These design patterns enable scalable sponsorship logic while defending against abuse, ensuring safe interaction between bundlers, users, and dApps.