Feature 3: Sign and Verify Posts

In this feature, we have introduced a couple new lnd endpoints, SignMessage and VerifyMessage. These can be useful in cases where one user wants to be certain that some content has not been altered by a third-party. Following our mantra of “Can’t Be Evil”, we want to ensure our users that we have not tampered with the posts they author. So when a user creates a post, we will sign the message using their node and store the signature. If another user wants to verify that the post has not been modified, they will verify the signature using their own node.

In addition to verifying signatures, we took this opportunity to remove the user-specified usernames for each post. Instead we use the node’s alias for this field.

Let’s go to the feat-3a branch to see what’s changed.

git checkout feat-3a

Replace username field with the node’s alias instead

To use the alias instead of a user-specified username, we needed to fetch the alias when the post is submitted to the backend and store it in the username field of the post. Then we need to remove the username field from the frontend flow that creates a post. As usual, we’ll start by updating the backend first then work our way up to the UI.

source: /backend/routes.ts

export const createPost = async (req: Request, res: Response) => {
 const { token, title, content } = req.body;
 const rpc = nodeManager.getRpc(token);

 const { alias } = await rpc.getInfo();

 const post = await db.createPost(alias, title, content);
 res.status(201).send(post);
};

We updated the createPost() route handler to get the alias using lnd’s getInfo() endpoint. Then we pass the alias to the db.createPost() function instead of the username received from the client. Notice that we no longer take the username from req.body as we do not require the client to send this value anymore. Instead, this handler expects to receive the node’s token, which will be automatically provided after the user connects their node.

source: /src/lib/api.ts

export const createPost = async (title: string, content: string) => {
 const request = { title, content };
 return await httpPost('posts', request);
};

On the frontend, we removed the username parameter in the API wrapper’s createPost() function since the backend no longer needs this info.

source: /src/store/store.ts

 createPost = async (title: string, content: string) => {
   this.clearError();
   try {
     await api.createPost(title, content);
     this.gotoPosts();
   } catch (err) {
     this.error = err.message;
   }
 };

In the mobx store, we removed the username parameter as well.

source: /src/pages/CreatepPost.tsx

const CreatePost: React.FC = () => {
 const store = useStore();

 const [title, setTitle] = useState('');
 const [content, setContent] = useState('');

 const handleSubmit = useCallback(
   async (e: React.FormEvent<HTMLElement>) => {
     e.preventDefault();
     store.createPost(title, content);
   },
   [title, content, store],
 );

 return (
   <Form onSubmit={handleSubmit}>
     <Card>
       <Card.Header>Create a new Post</Card.Header>
       <Card.Body>
         <Form.Group controlId="title">
           <Form.Label>Title</Form.Label>
           <Form.Control
             required
             value={title}
             onChange={e => setTitle(e.target.value)}
           />
         </Form.Group>
         <Form.Group controlId="title">
           <Form.Label>Content</Form.Label>
           <Form.Control
             required
             as="textarea"
             rows={8}
             value={content}
             onChange={e => setContent(e.target.value)}
           />
         </Form.Group>
       </Card.Body>
       <Card.Footer>
         <Row>
           <Col>
             <Button variant="outline-danger" onClick={store.gotoPosts}>
               Cancel
             </Button>
           </Col>
           <Col className="text-right">
             <Button variant="primary" type="submit">
               Submit
             </Button>
           </Col>
         </Row>
       </Card.Footer>
     </Card>
   </Form>
 );
};

Finally, in the CreatePost component, we removed the username state variable and associated field from the form.

Now when you visit the Create a Post screen in the browser, you will only see form fields to collect the Post’s title and content.

When you click submit, the new post will appear on the main screen with the subtitle “Posted by alice”. It set the username of the post to the alias of the node correctly.

Sign the message content when the post is created

Using lnd to sign a message is pretty straightforward. We just need to call the SignMessage endpoint with the message content in a specific format.

Checkout the feat-3b branch to see what’s changed.

git checkout feat-3b

source: /src/shared/types.ts

export interface Post {
 id: number;
 title: string;
 content: string;
 username: string;
 votes: number;
 signature: string;
 pubkey: string;
}

We’ll need to add a couple more fields to our Post model to store the signature and the node’s pubkey. These values are needed when another user wants to confirm that the signature is valid.

source: /backend/posts-db.ts

 async createPost(
   username: string,
   title: string,
   content: string,
   signature: string,
   pubkey: string,
 ) {
   // calculate the highest numeric id
   const maxId = Math.max(0, ...this._data.posts.map(p => p.id));

   const post: Post = {
     id: maxId + 1,
     title,
     content,
     username,
     votes: 0,
     signature,
     pubkey,
   };
   this._data.posts.push(post);

   await this.persist();
   this.emit(PostEvents.updated, post);
   return post;
 }

The createPost() function in the PostsDb class needed to be updated to add parameters for signature and pubkey, which are saved directly in the post.

Note: since we are modifying the schema of the Post model, you should delete the /db.json file now. Any posts that were created without these new fields will cause the app to throw errors when we run it the next time.

source: /backend/routes.ts

export const createPost = async (req: Request, res: Response) => {
 const { token, title, content } = req.body;
 const rpc = nodeManager.getRpc(token);

 const { alias, identityPubkey: pubkey } = await rpc.getInfo();
 // lnd requires the message to sign to be base64 encoded
 const msg = Buffer.from(content).toString('base64');
 // sign the message to obtain a signature
 const { signature } = await rpc.signMessage({ msg });

 const post = await db.createPost(alias, title, content, signature, pubkey);
 res.status(201).send(post);
};

The final step on the backend is to update the createPost() route handler to sign the message using lnd’s SignMessage endpoint, then pass the signature and pubkey to the database to store for later.

source: /db.json

 "posts": [
   {
     "id": 1,
     "title": "Moore's Law",
     "content": "SHA256 is not going to be broken by Moore’s law computational improvements in our lifetimes. If it’s going to get broken, it’ll be by some breakthrough cracking method.",
     "username": "alice",
     "votes": 2,
     "signature": "rn5h7rj497pryz8jmye5inqz4m1ykyg9e3js85a7zrbo9ugdh3bqo3zg8faam1tuxwdjupk4san8xayb5iqmc9jt1q7sni7hc6s1ce6b",
     "pubkey": "02dac3ee8de244753f50c6bc1ca5b51a0a71127061a995068115c723cef552aa72"
   }
 ],

Now if you create a new post, then open the /db.json file in your editor, you should see the new post listed with the pubkey and signature.

With these changes, the backend now supports signing all new posts when they are created. Our last step in this feature is to allow users to verify the signature using their node.

Add button to verify a post’s signature

Verifying a Post’s signature is very similar to signing a message. We’ll just need to make use of lnd’s VerifyMessage endpoint, giving it the post’s content and signature. It will return a valid flag set to true as well as the pubkey of the signing node which we can compare to the pubkey stored with the post record to confirm the signature is valid.

Checkout the feat-3c branch to see what’s changed.

git checkout feat-3c

source: /src/shared/types.ts

export interface Post {
 id: number;
 title: string;
 content: string;
 username: string;
 votes: number;
 signature: string;
 pubkey: string;
 verified: boolean;
}

We had to update the Post type to include a verified flag which will be false by default. We will switch it to true when another user verifies the content.

source: /backend/posts-db.ts

 async verifyPost(postId: number) {
   const post = this._data.posts.find(p => p.id === postId);
   if (!post) {
     throw new Error('Post not found');
   }
   post.verified = true;
   await this.persist();
   this.emit(PostEvents.updated, post);
 }

Next, we’ve added a new verifyPost() function to the PostsDb class. This function just sets the verified flag to true for a specified post.

source: /backend/index.ts

app.post('/api/posts/:id/verify', catchAsyncErrors(routes.verifyPost));

source: /backend/routes.ts

/**
* POST /api/posts/:id/verify
*/
export const verifyPost = async (req: Request, res: Response) => {
 const { id } = req.params;
 const { token } = req.body;
 // find the post
 const post = db.getPostById(parseInt(id));
 if (!post) throw new Error('Post not found');
 // find the node that's verifying this post
 const verifyingNode = db.getNodeByToken(token);
 if (!verifyingNode) throw new Error('Your node not found. Try reconnecting.');

 if (post.pubkey === verifyingNode.pubkey)
   throw new Error('You cannot verify your own posts!');

 const rpc = nodeManager.getRpc(verifyingNode.token);
 const msg = Buffer.from(post.content).toString('base64');
 const { signature } = post;
 const { pubkey, valid } = await rpc.verifyMessage({ msg, signature });

 if (!valid || pubkey !== post.pubkey) {
   throw new Error('Verification failed! The signature is invalid.');
 }

 db.verifyPost(post.id);
 res.send(post);
};

In the routes.ts file we’ve added the verifyPost() route handler and mapped it to the url in index.ts. This is where the bulk of the logic for verification is. There is a bunch going on here so let’s walk through it step by step.

1. Get the id of the post from the url and the verifying user’s token from the request body

2. Find the post in the DB using the id

3. Find the node record using the token of the user performing the verification

4. We do not want to allow a user to verify their own posts, so we compare the pubkey of the node that signed this post with the pubkey of the verifying user’s node. If they are the same, we throw an error

5. With the basic validation done, now we can get the RPC connection of the verifying node and call the VerifyMessage endpoint, passing it the base64 encoded message and the signature stored in the post record
6. The verifyMessage endpoint will return two values:

   1. valid is a boolean flag indicating if the signature is valid

   2. pubkey is the pubkey of the node that signed the message. It is recovered from the signature

7. To confirm the signature is valid, we must ensure that the valid flag is true and also that the response pubkey matches the pubkey of the node that created the post. It is possible that another node can sign this message and produce a valid signature, so we need to check both of these conditions.

8. Finally, if all of the verification checks pass, we call verifyPost() in the PostsDb class to set the post’s verified flag to true and return the updated post to the client

This completes all of the changes we need to make on the backend. Now let’s move to the frontend to implement verifying posts from the UI.

source: /src/lib/api.ts

export const verifyPost = async (postId: number) => {
 return await httpPost(`posts/${postId}/verify`);
};

In our API wrapper module, we added the verifyPost() function to make the http request to the backend.

source: /src/store/store.ts

 verifyPost = async (postId: number) => {
   this.clearError();
   try {
     const post = await api.verifyPost(postId);
     this._updatePost(post);
   } catch (err) {
     this.error = err.message;
   }
 };

In the mobx store, we added a new verifyPost() function which calls the API wrapper to make the http request, then updates the app state with the modified post record that is returned.

source: /src/components/PostCard.tsx

const PostCard: React.FC<Props> = ({ post }) => {
 return (
   <Card key={post.id} className="my-4">
     <Card.Body>
       <Card.Title>
         <strong>{post.title}</strong>
       </Card.Title>
       <Card.Subtitle className="mb-2 text-muted">
         Posted
         {post.signature && ' and signed '}
         by {post.username}
         {post.verified && (
           <Badge pill variant="success" className="ml-2">
             verified
           </Badge>
         )}
       </Card.Subtitle>
       <Card.Text>{post.content}</Card.Text>
     </Card.Body>
     <Card.Footer className="d-flex justify-content-between">
       <h5 className="mt-1">
         <Badge variant={post.votes ? 'primary' : 'light'}>{post.votes}</Badge> votes
       </h5>
       <span>
         <VerifyButton post={post} />
         <VoteButton post={post} />
       </span>
     </Card.Footer>
   </Card>
 );
};

In the PostCard component, we’ve made a few updates for post verification:

• In the post’s subtitle, we display that the post was signed by the user if there is a signature present.

• Also in the subtitle, it displays a badge with the text “verified” if the post’s verified flag is true.

• Next to the “Upvote” button we render a “Verify Signature” button. This is a new component that we’ll explain below.

source: /src/components/VerifyButton.tsx

const VerifyButton: React.FC<Props> = ({ post }) => {
 const store = useStore();

 const handleVerify = useCallback(() => {
   store.verifyPost(post.id);
 }, [store, post.id]);

 if (post.verified) {
   return null;
 }

 return (
   <Button variant="light" className="mr-3" onClick={handleVerify}>
     Verify Signature
   </Button>
 );
};

We created a new VerifyButton component which is pretty simple. If the post is not verified, it renders a button on the screen. When the button is clicked, it calls the store.verifyPost() function to initiate the signature verification.

Refresh the page in your browser and create a couple posts. You will now see the updates we’ve made. The post subtitles display that the posts are signed and there is a “Verify Signature” button. If you click on the button, you will receive an error message stating “You cannot verify your own posts!”. This is the intended behavior as we do not want to allow users to verify their own posts.

To test the Verify Signature feature, you’ll need to disconnect and reconnect using a different node, such as carol. Then you’ll be able to click on the “Verify Signature” button and see the “verified” badge now displayed.

We have now completed this feature. Next, we are going to take on our biggest improvement. We’ll implement making payments over Lightning in order to upvote a post.