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The b402 Protocol is an open standard for internet-native payments on the BNB Chain. It serves as the foundational layer of the broader b402 infrastructure and ecosystem, enabling seamless, programmable, and gasless payment experiences for users, developers, autonomous AI agents alike. At the core of the protocol is the b402 Relayer smart contract, an intermediary that powers gasless transactions across the network. The Relayer bridges the gap between tokens that do not natively support EIP-3009 (transferWithAuthorization) and applications that rely on it for meta-transactions. In simple terms, it allows users to authorize a payment off-chain with a signature, while the Relayer executes the payment on-chain on their behalf, eliminating the need for users to send transactions or pay gas directly.

b402 Relayer

⚠️ Note: This smart contract is currently unaudited. It is provided for testing and reference purposes only. We do not recommend using it in production environments.

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Why it matters

Most BEP-20 tokens, including major ones like BNB, USDT, or USD1 on the BNB Chain, do not implement EIP-3009, meaning they can’t natively process signed “transfer with authorization” operations. The B402 Relayer solves this by acting as a trusted middle layer that simulates EIP-3009 behavior using standard BEP-20 functions. This enables:
  1. Gasless payments: users sign a message; the relayer pays gas.
  2. Off-chain authorization: signatures replace manual on-chain approvals.
  3. Compatibility: works with any whitelisted ERC-20 token, even if it lacks EIP-3009 support.
  4. Security & auditability — verified with EIP-712 signatures and nonces.

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How it works

1

User Signs an Authorization

The user signs an EIP-712 structured message off-chain (using their wallet) that says: “I authorize B402 to transfer X tokens from me to Y, valid until Z.”This signed message includes:
  • The sender and receiver addresses
  • The token amount
  • Valid time window (validAfter, validBefore)
  • A unique nonce
  • The signature (v, r, s)
2

Relayer Executes On-Chain

When the Facilitator (B402’s off-chain service) receives the signed authorization from the client, it submits that authorization to the Relayer contract on-chain by calling transferWithAuthorization().Inside this function, the contract performs a series of strict checks before executing the transfer:
  1. Token Whitelist Check: The contract first verifies that the token is whitelisted. This prevents unknown or malicious ERC-20s from being used.
  2. Signature Verification: It then uses proper EIP-712 signature recovery to confirm the message was genuinely signed by the token owner (from). This ensures the relayer cannot forge authorizations.
  3. Authorization Validation: The contract confirms the signature’s time window is valid and that the nonce hasn’t been used or canceled before. This protects against replay attacks.
  4. Transfer Execution: Once validated, the relayer calls the actual token contract’s transferFrom() to move the specified amount from the sender to the recipient.
  5. Event Emission: Finally, it marks the nonce as used and emits AuthorizationUsed, creating an auditable on-chain record of the meta-transaction.
Through these steps, the Relayer securely mimics EIP-3009 behavior even for tokens that lack native support.