Anchor: How to ask the Nodejs backend to co-sign a transaction from the frontend without using Serum's Multisig?

Question

Ideally, I want my Nodejs backend (which secures a keypair) to co-sign a transaction coming from the frontend app (assuming the user will sign the transaction using his / her Solana wallet). The pubkey of the backend will be also included as a signer account to the anchor program.

I can't see any tutorial from Anchor doing this, hopefully it makes sense. I was thinking that the frontend will call a backend API passing down the serialize parameters, on which the backend will sign, and will return a signature to the frontend with its public key.

I have no idea on:

• What's the correct Anchor / Solana web3 to use to sign the transaction
• How can I add the returning signature (of the backend) to the transaction

I initially don't want to use Multisig with this approach as it will just overcomplicate things. Just need some confirmation from the backend before submitting the transaction.

Answer 1

You probably want to do something like this (backend):

// create the transaction, with user's public key as the feePayer

// Sign the transaction as the backend
// We must partial sign because the transaction still requires the user signature
transaction.partialSign(backendKeypair)

// Serialize the transaction and convert to base64 to return it
const serializedTransaction = transaction.serialize({
  // We will need the buyer to sign this transaction after it's returned to them
 requireAllSignatures: false
})
const base64 = serializedTransaction.toString('base64')

// return base64 to the frontend

Then from the frontend:

// make sure you're passing user's public key in this request
const response = await fetch(`/your-api/`, {
  method: 'POST',
  ...
})

// assuming your API returns JSON
const json = await response.json()

// Deserialize the transaction from the response
const transaction = Transaction.from(Buffer.from(json.transaction, 'base64'));
    
// have the user sign transaction using their connected wallet