# CCTP Message Format V1

This is CCTP V1 (Legacy) version. For the latest version, see [CCTP](https://developers.circle.com/cctp).

### CCTP V1 Message Header

The top-level message header format is standard for all messages passing through CCTP V1.

| Field               | Offset | Solidity Type | Length (bytes) | Description                                                                                                                 |
| ------------------- | -----: | ------------- | -------------: | --------------------------------------------------------------------------------------------------------------------------- |
| `version`           |      0 | uint32        |              4 | Version identifier - use 0 for CCTP V1                                                                                      |
| `sourceDomain`      |      4 | uint32        |              4 | Source domain ID                                                                                                            |
| `destinationDomain` |      8 | uint32        |              4 | Destination domain ID                                                                                                       |
| `nonce`             |     12 | uint64        |              8 | Unique message nonce (see [Sequential Nonces](#sequential-nonces))                                                          |
| `sender`            |     20 | bytes32       |             32 | Address of MessageTransmitter caller on source domain                                                                       |
| `recipient`         |     52 | bytes32       |             32 | Address to handle message body on destination domain                                                                        |
| `destinationCaller` |     84 | bytes32       |             32 | Address permitted to call MessageTransmitter on destination domain, or bytes32(0) if message can be received by any address |
| `messageBody`       |    116 | bytes         |        dynamic | Application-specific message to be handled by recipient                                                                     |

### Sequential Nonces

A message nonce is a unique identifier for a message that can only be used once on the destination domain. In CCTP V1, message nonces are implemented using **Sequential Nonces**, where 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.

Why we use `bytes32` type for addresses\
CCTP V1 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](https://github.com/circlefin/evm-cctp-contracts/blob/40111601620071988e94e39274c8f48d6f406d6d/src/messages/Message.sol#L145-L157) as a reference implementation for converting between address and `bytes32` in Solidity.

### CCTP V1 Message Body

The message format includes a dynamically sized `messageBody` field, used for application-specific messages. For example, TokenMessenger defines a [BurnMessage](https://github.com/circlefin/evm-cctp-contracts/blob/master/src/messages/BurnMessage.sol) with data related to cross-chain transfers.

| Field           | Offset | Solidity Type | Length (bytes) | Description                                                                            |
| --------------- | -----: | ------------- | -------------: | -------------------------------------------------------------------------------------- |
| `version`       |      0 | uint32        |              4 | Version identifier (0, for CCTP V1)                                                    |
| `burnToken`     |      4 | bytes32       |             32 | Address of burned token on source domain                                               |
| `mintRecipient` |     36 | bytes32       |             32 | Address to receive minted tokens on destination domain                                 |
| `amount`        |     68 | uint256       |             32 | Amount of burned tokens                                                                |
| `messageSender` |    100 | bytes32       |             32 | Address of caller of `depositForBurn` (or `depositForBurnWithCaller`) on source domain |


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