how can I send a signed transaction to the chain without a signer in the backend?

Question

I want to check the payment paid by the user in the chain and add it to database.

Frontend:

  const transactionString =
      "AQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAEDyIchCFWi8XOQK9v26xdqOA6+Nh0PxppQs5xbNaKQd7/0i+hG2L1v22b2JWcGVBnJMaqEN3yadwu9twp/uzdQZwAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA4VWxeJUJXETqt/CGxFeYTUs45eWde5bOPqThLZxVqEcBAgIAAQwCAAAAAOH1BQAAAAA=";

    const tx = Transaction.from(Buffer.from(transactionString, "base64"));
    tx.feePayer = anchorWallet.publicKey;

    await anchorWallet.signTransaction(tx);
    const signedData = tx.serialize().toString("base64");

    console.log(signedData);
    // AT9nK/UKWz+T/vhsli/hYVANRlVctUEPjJp+myjfeLNJqM5h982cyrQqbGlRnn/x3WwpJjOR3rI3JaxWfzuq+AUBAAEDyIchCFWi8XOQK9v26xdqOA6+Nh0PxppQs5xbNaKQd7/0i+hG2L1v22b2JWcGVBnJMaqEN3yadwu9twp/uzdQZwAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA4VWxeJUJXETqt/CGxFeYTUs45eWde5bOPqThLZxVqEcBAgIAAQwCAAAAAOH1BQAAAAA=

i send the signedData to the backend

app.post("/:publicKey/pay", async (c) => {
  const publicKey = new PublicKey(c.req.param("publicKey"));
  const tx = Transaction.from(Buffer.from(await c.req.text(), "base64"));

  const {
    value: { blockhash },
  } = await connection.getLatestBlockhashAndContext();

  tx.recentBlockhash = blockhash;

  const signature = await connection.sendRawTransaction(tx.serialize());
})

and I get the error "Trace: Signature verification failed". How can I solve this problem? Don't you already have the necessary signatures in the Transaction?

Answer 1

You need to do it in this order:

1. Attach recent blockhash
2. Sign transaction

You're doing it backwards, which will fail signature verification. This is because the recent blockhash is part of the message data being signed.

I'm not 100% sure, but it seems like the anchorWallet.signTransaction call in your frontend is already fetching the recent blockhash for you. So I think you can simply remove the assignment of the recent blockhash that you have in your backend, and you should be good.