Optimism Portal

Table of Contents

• Definitions
  • Custom Gas Token Flag
• Rationale
• Function Specification
  • Custom Gas Token Detection
  • donateETH
  • depositTransaction
• Security Considerations
  • Custom Gas Token Flag Immutability

Definitions

Custom Gas Token Flag

The Custom Gas Token Flag is determined by querying the SystemConfig contract's feature flags. The OptimismPortal determines if Custom Gas Token mode is active by checking if the CUSTOM_GAS_TOKEN feature is enabled in the SystemConfig contract, rather than maintaining its own local storage for this flag.

Rationale

The OptimismPortal's deposit logic must revert only for deposits with ETH value when Custom Gas Token mode is enabled to prevent ETH from acting as the native asset. Deposits without value (msg.value == 0) are permitted. Since the client side does not discern native asset supply creation, allowing ETH deposits with value would incorrectly imply that ETH can be minted in the chain.

The donateETH function is permitted to accept ETH even in Custom Gas Token mode, but such donations cannot be withdrawn through normal withdrawal mechanisms due to the restrictions on value-bearing withdrawals.

Function Specification

Custom Gas Token Detection

The OptimismPortal does not expose a public isCustomGasToken() function. Instead, it uses an internal _isUsingCustomGasToken() method that queries the SystemConfig contract to determine if the CUSTOM_GAS_TOKEN feature flag is enabled. External consumers should query the SystemConfig directly to determine the Custom Gas Token mode status.

donateETH

• Accepts ETH value without triggering a deposit to L2.

depositTransaction

• MUST revert if Custom Gas Token mode is active and msg.value > 0.

Security Considerations

Custom Gas Token Flag Immutability

The Custom Gas Token mode is controlled through the SystemConfig contract's feature flags. Once the CUSTOM_GAS_TOKEN feature is enabled in the SystemConfig, it should not be disabled in subsequent updates. Changing from custom gas token mode back to ETH mode could create inconsistencies in the chain's gas token handling and potentially lead to security vulnerabilities. The feature flag represents a fundamental configuration of the chain's gas token mechanism and should remain consistent throughout the chain's lifetime.