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USDC on Sophon

USDC on Sophon is implemented through a custom bridge implementation that enables seamless transfer of USDC between Ethereum (L1) and Sophon (L2). This bridge serves as an interim solution until ZKSync completes its interoperability implementation.

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Contract Addresses

• L2 USDC Token: 0x9Aa0F72392B5784Ad86c6f3E899bCc053D00Db4F
• L1 USDC Bridge: 0xf553E6D903AA43420ED7e3bc2313bE9286A8F987
• L2 USDC Bridge: 0x0f44bac3ec514be912aa4359017593b35e868d74

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Bridging USDC

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L1 → L2 (Ethereum to Sophon)

1. Call bridgehub.requestL2TransactionTwoBridges with the custom L1 bridge address as secondBridgeAddress
2. The bridgehub contract forwards the call to customBridgeL1.bridgehubDeposit
3. USDC is transferred from the user to the bridge contract
4. Sequencers pick up the event and call customL2Bridge.finalizeDeposit on L2
5. The L2 bridge mints USDC on Sophon

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L2 → L1 (Sophon to Ethereum)

1. Call customBridgeL2.withdraw on Sophon
2. Once the batch is sealed, call customL1Bridge.finalizeWithdrawal on Ethereum to complete the withdrawal

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Technical Details

The custom bridge implementation is based on the Shared Bridge code but modified to support the native USDC standard. The bridge consists of two main contracts:

• L1USDCBridge.sol: Handles operations on Ethereum
• L2USDCBridge.sol: Handles operations on Sophon

The L2 bridge contract has the MINTER role on the USDC token contract, allowing it to mint USDC tokens on Sophon when deposits are finalized.

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Bridged USDC Standard

The USDC implementation on Sophon follows Circle’s Bridged USDC Standard, which provides a secure and standardized way to implement bridged USDC with the possibility of upgrading to native USDC issuance in the future.

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Key Features

• Standardized Implementation: The bridged USDC token contract uses Circle’s audited FiatToken implementation, the same code that secures billions in USDC liquidity across other chains
• Upgradability: The implementation supports in-place upgrades to native USDC, allowing for a seamless transition without requiring time-consuming liquidity migration
• Security: Built on open-source, audited ERC-20 contract code that follows Circle’s security standards
• Compatibility: While bridged USDC is not initially compatible with CCTP (Cross-Chain Transfer Protocol), the upgrade path to native USDC would enable this functionality

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Bridge Requirements

The bridge implementation includes required functionality for potential future upgrades:

1. Pausable Bridging: The ability to pause USDC transfers to finalize the token supply
2. USDC Burning: Capability to burn locked USDC on the source chain during potential upgrades
3. Ownership Transfer: Support for secure transfer of contract ownership to Circle if an upgrade to native USDC is mutually agreed upon

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Role Management

The token contract implements Circle’s role-based access control system:

• Implementation Owner: Controls assignment of all roles and can be transferred to Circle for native upgrade
• ProxyAdmin: Manages proxy upgrades
• Minter Role: Assigned to the bridge contract for minting operations