Developer

USD₮0 is an Omnichain Fungible Token (OFT). The OFT standard allows tokens to move seamlessly across multiple blockchains using LayerZero's messaging protocol, ensuring a unified supply across all chains. USD₮0 follows this standard, leveraging LayerZero's infrastructure to enable secure and efficient cross-chain transfers while maintaining compatibility with existing USD₮ functionality.

1. Architecture Overview

The USD₮0 implementation separates token functionality from cross-chain messaging. This split enables independent upgrades of token and messaging components while maintaining consistent token behavior across chains.

Core Components

The implementation consists of three main components:

  1. OAdapterUpgradeable (on Ethereum):

    1. Implements LayerZero OFT functionality for Ethereum

    2. Handles both sending and receiving cross-chain messages

    3. Interfaces directly with the TetherToken contract on Ethereum

    4. Locks/Unlocks USD₮ for cross-chain transfers

  2. OUpgradeable (on other chains):

    1. Implements LayerZero OFT functionality for other chains

    2. Handles both sending and receiving cross-chain messages

    3. Interfaces with the TetherTokenOFTExtensoin

    4. Controls minting/burning for cross-chain transfers

  3. TetherTokenOFTExtension (on other chains):

    1. Offers mint/burn interface for the OFT

Component Interaction Diagram

The graphic illustrates how USD₮ and USD₮0 are transferred across Ethereum, Chain A, and Chain B:

Component Interaction Diagram

  1. Ethereum → Chain B:

    • USDT Adapter locks USDT on Ethereum.

    • A LayerZero message triggers USDT0 OFT on Chain B to mint equivalent USD₮0.

  2. Chain B → Chain A:

    • USDT0 OFT burns USD₮0 on Chain B.

    • A message triggers USDT0 OFT on Chain A to mint the equivalent.

  3. Chain A → Ethereum:

    • USDT0 OFT burns USD₮0 on Chain A.

    • A message instructs the USDT Adapter to unlock USD₮ on Ethereum.

The flow ensures consistent token supply across chains.

2. Interfaces Reference

Token Interfaces

The USD₮0 token implements the following standard interfaces:

  • ERC20

  • ERC20Permit (EIP-2612)

  • EIP-3009 (Gasless transfers)

Key public functions for integration:

interface IERC20 {
    function transfer(address to, uint256 amount) external returns (bool);
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
    function approve(address spender, uint256 amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function totalSupply() external view returns (uint256);
}

interface IERC20Permit {
    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;
}

// EIP-3009 functions for gasless transfers
interface IEIP3009 {
    function transferWithAuthorization(
        address from,
        address to,
        uint256 value,
        uint256 validAfter,
        uint256 validBefore,
        bytes32 nonce,
        bytes memory signature
    ) external;

    function receiveWithAuthorization(
        address from,
        address to,
        uint256 value,
        uint256 validAfter,
        uint256 validBefore,
        bytes32 nonce,
        bytes memory signature
    ) external;
}

OFT Interfaces

The OFT implementation provides cross-chain transfer functionality through LayerZero:

interface IOFT {
    struct SendParam {
        uint32 dstEid;          // Destination endpoint ID
        bytes32 to;             // Recipient address
        uint256 amountLD;       // Amount to send
        uint256 minAmountLD;    // Minimum amount to receive
        bytes extraOptions;     // Additional options
        bytes composeMsg;       // Optional composed message
        bytes oftCmd;           // OFT-specific command
    }

    struct MessagingFee {
        uint256 nativeFee;     // Fee in native gas
        uint256 lzTokenFee;    // Fee in LZ token
    }

    struct OFTReceipt {
        uint256 amountSentLD;      // Amount sent
        uint256 amountReceivedLD;  // Amount received
    }

    // Get expected received amount
    function quoteOFT(SendParam calldata sendParam)
        external view returns (
            OFTLimit memory,          // Min/max amounts
            OFTFeeDetail[] memory,    // Fee breakdown
            OFTReceipt memory        // Expected amounts
        );

    // Get messaging fee
    function quoteSend(SendParam calldata sendParam, bool payInLzToken)
        external view returns (MessagingFee memory);

    // Execute the cross-chain transfer
    function send(
        SendParam calldata sendParam,
        MessagingFee calldata fee,
        address refundAddress
    ) external payable;
}

The OFT interface is consistent across all chains, whether using OAdapterUpgradeable on Ethereum or OUpgradeable on other chains. The only difference is that on Ethereum, users need to approve the OFT adapter to spend their USD₮ before calling send.

Example

The following example shows how to bridge USD₮ from Ethereum to Arbitrum, including necessary approvals and parameter handling.

import { ethers } from "ethers";

const USDT_ADDRESS = "0xdAC17F958D2ee523a2206206994597C13D831ec7";
const OFT_ADDRESS = "0x6C96dE32CEa08842dcc4058c14d3aaAD7Fa41dee";
const ARB_EID = 30110;
const MAX_UINT256 = ethers.constants.MaxUint256;

const OFT_ABI = [
  "function quoteOFT(tuple(uint32,bytes32,uint256,uint256,bytes,bytes,bytes)) view returns (tuple(uint256,uint256), tuple(int256,string)[], tuple(uint256,uint256))",
  "function quoteSend(tuple(uint32,bytes32,uint256,uint256,bytes,bytes,bytes), bool) view returns (tuple(uint256,uint256))",
  "function send(tuple(uint32,bytes32,uint256,uint256,bytes,bytes,bytes), tuple(uint256,uint256), address) payable returns (tuple(bytes32,uint64,tuple(uint256,uint256)), tuple(uint256,uint256))",
];

async function sendUSDT0ToArbitrum(
  signer: ethers.Signer,
  amount: string,
  recipient: string
) {
  const usdt = new ethers.Contract(
    USDT_ADDRESS,
    ["function approve(address,uint256)"],
    signer
  );
  const oft = new ethers.Contract(OFT_ADDRESS, OFT_ABI, signer);

  const amountWei = ethers.utils.parseUnits(amount, 6);

  // Approve max amount
  await usdt.approve(OFT_ADDRESS, MAX_UINT256);

  const sendParam = [
    ARB_EID,
    ethers.utils.hexZeroPad(recipient, 32),
    amountWei,
    0,
    "0x",
    "0x",
    "0x",
  ];

  const [, , oftReceipt] = await oft.callStatic.quoteOFT(sendParam);
  sendParam[3] = oftReceipt[1];
  const msgFee = await oft.callStatic.quoteSend(sendParam, false);

  const tx = await oft.send(sendParam, msgFee, recipient, { value: msgFee[0] });

  return tx;
}

3. Security Configuration (DVNs)

USD₮0 utilizes a dual-DVN security configuration requiring verification from:

  • LayerZero DVN

  • USDT0 DVN

Both DVNs must verify the payloadHash before a cross-chain message can be committed for execution. This setup ensures enhanced security through independent verification of all cross-chain transfers.

For detailed information about LayerZero DVNs and security stacks, refer to: https://docs.layerzero.network/v2/home/modular-security/security-stack-dvns

4. Legacy Mesh

The Legacy Mesh is a cross-chain liquidity network for USDT that connects legacy deployments on Ethereum, Arbitrum, Celo, Tron, and TON. It enables USDT transfers across chains without minting or burning. Instead, it uses a credit-based system where liquidity is locked and unlocked between smart contract pools on each chain.

At its core is the UsdtOFT contract, which implements LayerZero’s IOFT interface for compatibility with standard OFT tooling. Unlike standard OFT contracts, UsdtOFT does not alter token supply—it moves USDT by crediting and debiting pool balances.

Key Mechanics

  • Fee-based Transfers: A small fee (in basis points) is deducted from each transfer. The feeBps variable defines the rate.

  • Interface Support:

    • quoteOFT(): Returns transfer limits, expected amounts, and fees.

    • quoteSend(): Returns LayerZero messaging fees.

    • send(): Executes the cross-chain transfer and applies fees.

This mechanism enables seamless interoperability across non-upgradeable USDT deployments.

Please note: When an upgrade is processed, the entire Legacy Mesh infrastructure smart contracts are migrated as a whole. If your application integrates Legacy Mesh directly at the smart contract level, we recommend reaching out to [email protected]. This ensures you have a direct communication channel and remain fully updated on the latest changes.

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