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Message Header
The top-level message header format is standard for all messages passing through CCTP:
| Field | Bytes | Type | Index | Description |
|---|---|---|---|---|
| version | 4 | uint32 | 0 | Version identifier |
| sourceDomain | 4 | uint32 | 4 | Source domain id |
| destinationDomain | 4 | uint32 | 8 | Destination domain id |
| nonce | 8 | uint64 | 12 | Unique message nonce |
| sender | 32 | bytes32 | 20 | Address of MessageTransmitter caller on source domain |
| recipient | 32 | bytes32 | 52 | Address to handle message body on destination domain |
| destinationCaller | 32 | bytes32 | 84 | Address permitted to call MessageTransmitter on destination domain, or bytes32(0) if message can be received by any address |
| messageBody | dynamic | bytes | 116 | Application-specific message to be handled by recipient |
Why we use bytes32 type for addresses
CCTP is built to support EVM chains, which use 20 byte addresses, and non-EVM chains, many of which use 32 byte addresses. We provide a Message.sol library as a reference implementation for converting between address and bytes32 in Solidity.
Message Body
The message format includes a dynamically sized messageBody field, used for application-specific messages. For example, TokenMessenger defines a BurnMessage with data related to cross-chain transfers:
| Field | Bytes | Type | Index | Description |
|---|---|---|---|---|
| version | 4 | uint32 | 0 | Version identifier |
| burnToken | 32 | bytes32 | 4 | Address of burned token on source domain |
| mintRecipient | 32 | bytes32 | 36 | Address to receive minted tokens on destination domain |
| amount | 32 | uint256 | 68 | Amount of burned tokens |
| messageSender | 32 | bytes32 | 100 | Address of caller of depositForBurn (or depositForBurnWithCaller) on source domain |
Message Nonce
A message nonce is a unique identifier for a message that can only be used once on the destination domain. The next available nonce on a source domain is an integer. On the destination domain, messages can be received in any order, and used nonces are stored as a hash of the source domain and nonce integer value.
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