Submit Message to Private Topic

Summary

In the previous tutorial, "Submit Your First Message," you have learned how to submit a message to a public topic. It means anyone can send a message to the topic you created because you didn't set a Submit Key.

When setting a Submit Key, your topic becomes a private topic because each message needs to be signed by the Submit Key. Therefore, you can control who can submit messages to your topic. Of course, the data is still public, as is all data on a public ledger, but we say the topic is private because the topic is restricted by who can submit messages to it.

Prerequisites

We recommend you complete the "Submit Your First Message" tutorial here to get a basic understanding of the Hedera Consensus Service. This example does not build upon the previous examples.

✅ You can find a full code check for this tutorial at the bottom of this page.

1. Create a private topic

To create a private topic, you will use setSubmitKey() to set a Submit Key. This key needs to sign all messages someone sends to the topic. A message will be rejected if you don't sign the message or sign with an incorrect key. The cost of creating a private topic is the same as a public topic: $0.01.

// Create a new topic
TransactionResponse txResponse = new TopicCreateTransaction()
   .setSubmitKey(myPrivateKey.getPublicKey())
   .execute(client);

// Get the receipt
TransactionReceipt receipt = txResponse.getReceipt(client);
        
// Get the topic ID
TopicId topicId = receipt.topicId;

// Log the topic ID
System.out.println("Your topic ID is: " +topicId);

// Wait 5 seconds between consensus topic creation and subscription creation
Thread.sleep(5000);

2. Subscribe to a topic

The code used to subscribe to a public or private topic doesn't change. Anyone can listen to the messages you send to your private topic. You need to provide the TopicMessageQuery() with your topic ID to subscribe to it. 

// Subscribe to the topic
new TopicMessageQuery()
    .setTopicId(topicId)
    .subscribe(client, resp -> {
            String messageAsString = new String(resp.contents, StandardCharsets.UTF_8);
            System.out.println(resp.consensusTimestamp + " received topic message: " + messageAsString);
    });

3. Submit a message

Now you are ready to submit a message to your private topic. To do this, you will use TopicMessageSubmitTransaction(). However, you need to sign this transaction with your Submit Key. The cost for sending a message to a private topic is the same as a public topic: $0.0001. 

// Send message to private topic
TransactionResponse submitMessage = new TopicMessageSubmitTransaction()
      .setTopicId(topicId)
      .setMessage("Submitkey set!")
      .freezeWith(client)
      .sign(myPrivateKey)
      .execute(client)

// Get the receipt of the transaction
TransactionReceipt receipt2 = submitMessage.getReceipt(client);

// Prevent the main thread from exiting so the topic message can be returned and printed to the console
Thread.sleep(30000);

To conclude: The total cost to create a topic and send a message to it is $0.0101.

Code Check ✅

Java
import com.hedera.hashgraph.sdk.*;
import io.github.cdimascio.dotenv.Dotenv;

import java.nio.charset.StandardCharsets;
import java.util.concurrent.TimeoutException;

public class CreateTopicTutorial {
    public static void main(String[] args) throws TimeoutException, PrecheckStatusException, ReceiptStatusException, InterruptedException {

        // Grab your Hedera testnet account ID and private key
        AccountId myAccountId = AccountId.fromString(Dotenv.load().get("MY_ACCOUNT_ID"));
        PrivateKey myPrivateKey = PrivateKey.fromString(Dotenv.load().get("MY_PRIVATE_KEY"));

        // Build your Hedera client
        Client client = Client.forTestnet();
        client.setOperator(myAccountId, myPrivateKey);

        // Create a new topic
        TransactionResponse txResponse = new TopicCreateTransaction()
                .setSubmitKey(myPrivateKey.getPublicKey())
                .execute(client);

        // Get the receipt
        TransactionReceipt receipt = txResponse.getReceipt(client);

        // Get the topic ID
        TopicId topicId = receipt.topicId;

        // Log the topic ID
        System.out.println("Your topic ID is: " +topicId);

        // Wait 5 seconds between consensus topic creation and subscription creation
        Thread.sleep(5000);

        // Subscribe to the topic
        new TopicMessageQuery()
                .setTopicId(topicId)
                .subscribe(client, resp -> {
                    String messageAsString = new String(resp.contents, StandardCharsets.UTF_8);
                    System.out.println(resp.consensusTimestamp + " received topic message: " + messageAsString);
                });

        // Send message to private topic
        TransactionResponse submitMessage = new TopicMessageSubmitTransaction()
              .setTopicId(topicId)
              .setMessage("Submitkey set!")
              .freezeWith(client)
              .sign(myPrivateKey)
              .execute(client)

        // Get the receipt of the transaction
        TransactionReceipt receipt2 = submitMessage.getReceipt(client);

        // Wait before the main thread exits to return the topic message to the console
        Thread.sleep(30000);
    }
}
JavaScript
console.clear();
require("dotenv").config();
const {
  AccountId,
  PrivateKey,
  Client,
  TopicCreateTransaction,
  TopicMessageQuery,
  TopicMessageSubmitTransaction,
} = require("@hashgraph/sdk");

// Grab the OPERATOR_ID and OPERATOR_KEY from the .env file
const myAccountId = process.env.MY_ACCOUNT_ID;
const myPrivateKey = process.env.MY_PRIVATE_KEY;

// Build Hedera testnet and mirror node client
const client = Client.forTestnet();

// Set the operator account ID and operator private key
client.setOperator(myAccountId, myPrivateKey);

async function submitPrivateMessage() {
  // Create a new topic
  let txResponse = await new TopicCreateTransaction()
    .setSubmitKey(myPrivateKey.publicKey)
    .execute(client);

  // Grab the newly generated topic ID
  let receipt = await txResponse.getReceipt(client);
  let topicId = receipt.topicId;
  console.log(`Your topic ID is: ${topicId}`);

  // Wait 5 seconds between consensus topic creation and subscription creation
  await new Promise((resolve) => setTimeout(resolve, 5000));

  // Create the topic
  new TopicMessageQuery()
    .setTopicId(topicId)
    .subscribe(client, null, (message) => {
      let messageAsString = Buffer.from(message.contents, "utf8").toString();
      console.log(
        `${message.consensusTimestamp.toDate()} Received: ${messageAsString}`
      );
    });

  // Send message to private topic
  let submitMsgTx = await new TopicMessageSubmitTransaction({
    topicId: topicId,
    message: "Submitkey set!",
  })
  .freezeWith(client)
  .sign(myPrivateKey);

  let submitMsgTxSubmit = await submitMsgTx.execute(client);
  let getReceipt = await submitMsgTxSubmit.getReceipt(client);

  // Get the status of the transaction
  const transactionStatus = getReceipt.status;
  console.log("The message transaction status: " + transactionStatus.toString());
}

submitPrivateMessage();
Go
package main

import (
	"fmt"
	"os"
	"time"

	"github.com/hashgraph/hedera-sdk-go/v2"
	"github.com/joho/godotenv"
)

func main() {

	// Loads the .env file and throws an error if it cannot load the variables from that file corectly
	err := godotenv.Load(".env")
	if err != nil {
		panic(fmt.Errorf("Unable to load enviroment variables from .env file. Error:\n%v\n", err))
	}

	// Grab your testnet account ID and private key from the .env file
	myAccountId, err := hedera.AccountIDFromString(os.Getenv("MY_ACCOUNT_ID"))
	if err != nil {
		panic(err)
	}

	myPrivateKey, err := hedera.PrivateKeyFromString(os.Getenv("MY_PRIVATE_KEY"))
	if err != nil {
		panic(err)
	}

	// Create your testnet client
	client := hedera.ClientForTestnet()
	client.SetOperator(myAccountId, myPrivateKey)

	// Create a new topic
	transactionResponse, err := hedera.NewTopicCreateTransaction().
		SetSubmitKey(myPrivateKey.PublicKey()).
		Execute(client)

	if err != nil {
		println(err.Error(), ": error creating topic")
		return
	}

	// Get the topic create transaction receipt
	transactionReceipt, err := transactionResponse.GetReceipt(client)

	if err != nil {
		println(err.Error(), ": error getting topic create receipt")
		return
	}

	// Get the topic ID from the transaction receipt
	topicID := *transactionReceipt.TopicID

	// Log the topic ID to the console
	fmt.Printf("topicID: %v\n", topicID)

	//Create the query to subscribe to a topic
	_, err = hedera.NewTopicMessageQuery().
		SetTopicID(topicID).
		Subscribe(client, func(message hedera.TopicMessage) {
			fmt.Println(message.ConsensusTimestamp.String(), "received topic message ", string(message.Contents), "\r")
		})
        
        // Prepare message to send to private topic
	submitMessageTx, err := hedera.NewTopicMessageSubmitTransaction().
		SetMessage([]byte("Submitkey set!")).
		SetTopicID(topicID).
		FreezeWith(client)
	
	if err != nil {
		println(err.Error(), ": error freezing topic message submit transaction")
		return
	}
	
	// Sign message with submit key
	submitMessageTx.Sign(myPrivateKey)
	
	// Submit message
	submitTxResponse, err := submitMessageTx.Execute(client)
	if err != nil {
		println(err.Error(), ": error submitting to topic")
		return
	}
	
	// Get the receipt of the transaction
	receipt, err := submitTxResponse.GetReceipt(client)
	
	// Get the transaction status
	transactionStatus := receipt.Status
	fmt.Println("The message transaction status " + transactionStatus.String())
	
	// Prevent the program from exiting to display the message from the mirror node to the console
	time.Sleep(30000)
    }

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Writer: Michiel, Developer Advocate

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Editor: Krystal, Technical Writer

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