sign-message.md

Sign Message

Once the web application is connected to Blocto wallet, it can sign the message on behalf of the user, with the user's permission.

In order to sign a message, the web application must:

• Create a sign message request payload.
• Have it be signed by the user's Blocto wallet.

Sign a message

const response = await aptos.signMessage({
  message: "Hello world",
  nonce: "foo", // a unique identifier for the request
})
const { signature } = response // an array of signatures of the multi-sig account

Verify signatures

After getting the signature, you might want to verify it to see whether it's valid.

For this purpose, we need to figure out the keys that we used to sign the message since Blocto wallet creates multi-signers account for you.

By checking with the bitmap in the response you got, you can get the public keys that we need for verifying the signatures.

// a 4-byte (32 bits) bit-vector of length N
const { bitmap } = response

const input = new Uint8Array(bitmap)
// Getting an array which marks the keys signing the message with 1, while marking 0 for the keys not being used.
const bits = Array.from(input).flatMap((n) =>
  Array.from({ length: 8 }).map((_, i) => (n >> i) & 1)
)
// Filter out indexes of the keys we need
const index = bits.map((_, i) => i).filter((i) => bits[i]) 

const { publicAccount: { publicKey } } = bloctoSDK.aptos
// The filter result is an array of the keys which we need for verifying the signatures
const publicKeys = publicKey.filter((_: string, i: number) => index.includes(i))

Here we use tweetnacl for verifying the message. For each signature, we need to verify with the corresponding public key we just got by reading bitmap. Every signature should pass the verification.

import nacl from 'tweetnacl';

// ...
const { fullMessage, signature } = response

for (let i = 0; i < signature.length; i++) {
  const isVerfied = nacl.sign.detached.verify(
    Buffer.from(fullMessage),
    Buffer.from(signature[i], 'hex'),
    Buffer.from(publicKeys[i], 'hex')
  ); // `isVerfied` should be `true` for every signature
}

Sample Code

{% embed url="https://codesandbox.io/s/github/blocto/blocto-sdk-examples/tree/main/src/aptos/with-blocto-sign?file=/src/App.js" %}