> ## Documentation Index
> Fetch the complete documentation index at: https://developers.circle.com/llms.txt
> Use this file to discover all available pages before exploring further.

# CCTP message format V1

> Format arbitrary and application-specific messages using CCTP V1

<Warning>
  **This is CCTP V1 (Legacy) version. For the latest version, see [CCTP](/cctp)**.
</Warning>

## 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.

<Note>
  **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.
</Note>

## 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 |