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Overview

The MessageTransmitter contract is the core component of CCTP that handles cross-chain message transmission. It manages message dispatch, receipt, and validation, ensuring secure communication between different blockchain domains.

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State Variables

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localDomain
uint32
required
Domain of the chain on which the contract is deployed. This is an immutable value set during contract deployment.
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version
uint32
required
Message format version. This is an immutable value that defines the structure of messages handled by this contract.
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maxMessageBodySize
uint256
required
Maximum size of message body in bytes. This value can be updated by the contract owner to accommodate larger messages.
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nextAvailableNonce
uint64
required
Next available nonce from this source domain. This counter increments with each message sent to ensure uniqueness.

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Functions

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sendMessage

Sends a message to the destination domain and recipient. 
function sendMessage(
    uint32 destinationDomain,
    bytes32 recipient,
    bytes calldata messageBody
) external returns (uint64)
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destinationDomain
uint32
required
Domain of destination chain
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recipient
bytes32
required
Address of message recipient on destination chain as bytes32
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messageBody
bytes
required
Raw bytes content of message
Returns: uint64 - The nonce reserved by the message Behavior:
  • Increments the nonce counter
  • Formats the message with version, domains, nonce, sender, and recipient
  • Emits a MessageSent event with the complete message
  • Reverts if the contract is paused
  • Reverts if message body exceeds maxMessageBodySize

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sendMessageWithCaller

Sends a message to the destination domain and recipient, specifying a destinationCaller that is authorized to receive the message on the destination domain. 
function sendMessageWithCaller(
    uint32 destinationDomain,
    bytes32 recipient,
    bytes32 destinationCaller,
    bytes calldata messageBody
) external returns (uint64)
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destinationDomain
uint32
required
Domain of destination chain
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recipient
bytes32
required
Address of message recipient on destination chain as bytes32
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destinationCaller
bytes32
required
Caller on the destination domain, as bytes32. Must be nonzero.
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messageBody
bytes
required
Raw bytes content of message
Returns: uint64 - The nonce reserved by the message
If the destinationCaller does not represent a valid address, it will not be possible to broadcast the message on the destination domain. This is an advanced feature - the standard sendMessage() should be preferred for most use cases.

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receiveMessage

Receives a message from a source domain. Messages with a given nonce can only be broadcast once for a (sourceDomain, destinationDomain) pair. 
function receiveMessage(
    bytes calldata message,
    bytes calldata attestation
) external returns (bool success)
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message
bytes
required
Message bytes containing:
  • version (4 bytes, uint32)
  • sourceDomain (4 bytes, uint32)
  • destinationDomain (4 bytes, uint32)
  • nonce (8 bytes, uint64)
  • sender (32 bytes, bytes32)
  • recipient (32 bytes, bytes32)
  • destinationCaller (32 bytes, bytes32)
  • messageBody (dynamic bytes)
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attestation
bytes
required
Concatenated 65-byte signature(s) of message, in increasing order of the attester address recovered from signatures. Must contain exactly the threshold number of valid signatures.
Returns: bool - true if successful Message Format:
FieldBytesTypeIndex
version4uint320
sourceDomain4uint324
destinationDomain4uint328
nonce8uint6412
sender32bytes3220
recipient32bytes3252
destinationCaller32bytes3284
messageBodydynamicbytes116
Validation:
  • Verifies attestation signatures
  • Validates message format
  • Checks destination domain matches local domain
  • Validates destination caller (if specified)
  • Validates message version
  • Ensures nonce has not been used before
  • Marks nonce as used upon successful receipt
  • Calls handleReceiveMessage() on the recipient contract
Attestations must have signatures in increasing order of attester address. If signatures are not in order, or if there are duplicate or incorrect number of signatures, verification will fail.

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replaceMessage

Replaces a message with a new message body and/or destination caller. Allows the sender of a previous message to send a new message with the same nonce. 
function replaceMessage(
    bytes calldata originalMessage,
    bytes calldata originalAttestation,
    bytes calldata newMessageBody,
    bytes32 newDestinationCaller
) external
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originalMessage
bytes
required
Original message to replace
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originalAttestation
bytes
required
Valid attestation of originalMessage
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newMessageBody
bytes
required
New message body of replaced message
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newDestinationCaller
bytes32
required
The new destination caller, which may be the same as the original destination caller, a new destination caller, or an empty destination caller (bytes32(0), indicating that any destination caller is valid)
Behavior:
  • Validates the original attestation signatures
  • Verifies msg.sender matches the sender of the original message
  • Verifies the source domain of the original message matches local domain
  • Reuses the original message’s nonce
  • Emits a new MessageSent event with the replacement message
The new message will reuse the original message’s nonce. For a given nonce, all replacement message(s) and the original message are valid to broadcast on the destination domain, until the first message at the nonce confirms, at which point all others are invalidated.

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setMaxMessageBodySize

Sets the maximum message body size. Only callable by the contract owner. 
function setMaxMessageBodySize(
    uint256 newMaxMessageBodySize
) external
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newMaxMessageBodySize
uint256
required
New maximum message body size, in bytes
This value should not be reduced without good reason, to avoid impacting users who rely on large messages.

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Events

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MessageSent

Emitted when a new message is dispatched. 
event MessageSent(bytes message)
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message
bytes
Raw bytes of the complete message including all headers and body

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MessageReceived

Emitted when a new message is received. 
event MessageReceived(
    address indexed caller,
    uint32 sourceDomain,
    uint64 indexed nonce,
    bytes32 sender,
    bytes messageBody
)
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caller
address
Caller (msg.sender) on destination domain
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sourceDomain
uint32
The source domain this message originated from
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nonce
uint64
The nonce unique to this message
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sender
bytes32
The sender of this message
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messageBody
bytes
Message body bytes

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MaxMessageBodySizeUpdated

Emitted when the maximum message body size is updated. 
event MaxMessageBodySizeUpdated(uint256 newMaxMessageBodySize)
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newMaxMessageBodySize
uint256
New maximum message body size, in bytes

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