ZK implementation on Ton

```
// Struct to represent a cross-chain message
struct CrossChainMessage {
    messageHash: Int256;
    sourceChainId: Int256;
    destinationChainId: Int256;
    sender: Address;
    recipient: Address;
    payload: Slice;
    zkProof: Slice;
    verified: Bool;
    executed: Bool;
}

// ZK Bridge Messenger Contract
contract ZKBridgeMessenger with Deployable, Ownable {
    owner: Address;
    verifierPublicKey: Int256;
    messages: map<Int256, CrossChainMessage>;
    processedMessages: map<Int256, Bool>;

    // Events for tracking message lifecycle
    event MessageReceived(messageHash: Int256, sourceChainId: Int256, destinationChainId: Int256);
    event MessageVerified(messageHash: Int256, verified: Bool);
    event MessageExecuted(messageHash: Int256, recipient: Address);

    // Constructor
    init(verifierKey: Int256) {
        self.owner = sender();
        self.verifierPublicKey = verifierKey;
    }

    // Receive a cross-chain message with ZK proof
    receive(msg: InternalMessage) {
        let messageHash: Int256 = self.generateMessageHash(
            msg.sourceChainId, 
            msg.destinationChainId, 
            msg.sender, 
            msg.recipient, 
            msg.payload
        );

        // Prevent message replay
        require(!self.processedMessages.get(messageHash)!, "Message already processed");

        // Store message
        let message: CrossChainMessage = CrossChainMessage {
            messageHash: messageHash,
            sourceChainId: msg.sourceChainId,
            destinationChainId: msg.destinationChainId,
            sender: msg.sender,
            recipient: msg.recipient,
            payload: msg.payload,
            zkProof: msg.zkProof,
            verified: false,
            executed: false
        };

        self.messages.set(messageHash, message);
        self.processedMessages.set(messageHash, true);

        emit MessageReceived(messageHash, msg.sourceChainId, msg.destinationChainId);
    }

    // Verify the zero-knowledge proof for a message
    fun verifyMessage(messageHash: Int256) {
        // Retrieve message
        let message: CrossChainMessage = self.messages.get(messageHash)!;
        
        require(!message.verified, "Message already verified");

        // Placeholder for ZK proof verification
        // Replace with actual circuit verification logic
        let proofValid: Bool = self.verifyZKProof(
            message.zkProof, 
            message.payload, 
            self.verifierPublicKey
        );

        // Update message verification status
        message.verified = proofValid;
        self.messages.set(messageHash, message);

        emit MessageVerified(messageHash, proofValid);
    }

    // Execute a verified message
    fun executeMessage(messageHash: Int256) {
        // Retrieve message
        let message: CrossChainMessage = self.messages.get(messageHash)!;

        require(message.verified, "Message not verified");
        require(!message.executed, "Message already executed");
        require(sender() == message.recipient, "Unauthorized recipient");

        // Mark message as executed
        message.executed = true;
        self.messages.set(messageHash, message);

        // Execute payload (implement cross-chain logic)
        self.executePayload(message.payload, message.recipient);

        emit MessageExecuted(messageHash, message.recipient);
    }

    // Generate unique message hash
    fun generateMessageHash(
        sourceChainId: Int256, 
        destinationChainId: Int256, 
        sender: Address, 
        recipient: Address, 
        payload: Slice
    ): Int256 {
        // Implement a secure hash generation method
        // This is a placeholder - replace with a more robust hashing mechanism
        return sha256(
            sourceChainId.toString() + 
            destinationChainId.toString() + 
            sender.toString() + 
            recipient.toString() + 
            payload.toString()
        );
    }

    // Placeholder ZK proof verification
    fun verifyZKProof(
        proof: Slice, 
        payload: Slice, 
        publicKey: Int256
    ): Bool {
        // IMPORTANT: Replace with actual ZK circuit verification
        // This is a mock implementation
        return sha256(proof.toString()) == sha256(payload.toString() + publicKey.toString());
    }

    // Execute payload on the recipient contract
    fun executePayload(payload: Slice, recipient: Address) {
        // Implement cross-chain payload execution
        // This might involve sending a message to another contract or performing specific actions
        // Placeholder implementation
        send(SendParameters{
            to: recipient,
            value: 0,
            mode: SendMode.CARRY_ALL_REMAINING_MESSAGE_VALUE,
            body: payload
        });
    }

    // Owner can update verifier public key
    receive(msg: UpdateVerifierKey) {
        require(sender() == self.owner, "Unauthorized");
        self.verifierPublicKey = msg.newPublicKey;
    }

    // Fallback function to handle unexpected messages
    fallback() {
        // Reject unexpected messages
        throw();
    }
}

// Message for updating verifier public key
message UpdateVerifierKey {
    newPublicKey: Int256;
}

// Internal message structure for cross-chain communication
struct InternalMessage {
    sourceChainId: Int256;
    destinationChainId: Int256;
    sender: Address;
    recipient: Address;
    payload: Slice;
    zkProof: Slice;
}
```// Some code