How to Sign a Transaction with Multiple Signers

Question

I have a transaction that uses 2 signers(the user and the Keypair from the backend). It signs from the Backend and returns a serialized transaction for the user to sign. However, I am getting this Error

`Error: failed to send transaction: Transaction signature verification failure

at Connection.sendEncodedTransaction(node_modules/@solana/web3.js/src/connection.ts:5054:13)

at processTicksAndRejections(node:internal/process/task_queues:96:5) 

at Connection.sendRawTransaction(node_modules/@solana/web3.js/src/connection.ts:5013:20)`.

This is my code snippet

  const tx = await program.methods.create(new anchor.BN(3))
  .accounts({
    payer: provider.wallet.publicKey,
    backend: backend.publicKey,
    systemProgram: SystemProgram.programId
  }).transaction();

  tx.recentBlockhash = (
    await program.provider.connection.getLatestBlockhash()
  ).blockhash;

  tx.sign(backend);
  const serializedTx = tx.serialize({ requireAllSignatures: false });

  // Deserialize it and then sign with provider wallet
  let recoveredTransaction: Transaction | undefined = Transaction.from(Buffer.from(serializedTx));
  
  recoveredTransaction = await provider.wallet.signTransaction(recoveredTransaction);
  recoveredTransaction.feePayer = provider.wallet.publicKey;

  const final_tx =
      (await provider.connection.sendRawTransaction(
          recoveredTransaction.serialize({ verifySignatures: false, requireAllSignatures: false }),
      ));

  await provider.connection.confirmTransaction(final_tx, "confirmed");

Answer 1

You cannot modify the transaction at all after signing it. Adding the instructions, setting the feepayer, and setting the blockhash need to be done prior to any signatures.

Signing the transaction should be the last step before sending it to RPC.

Answer 2

The root cause of this Error seems to be that when the backend signs the transaction, If a feePayer was not specified, the transaction automatically sets the feePayer to the backend publickey. Hence when the feePayer is later modified to the user publickey and the user signs the transaction after deserializing the transaction data, It seems the transaction data is also changed and does not produce the same Initial serialized transaction which consequently causes the Error: failed to send transaction: Transaction signature verification failure error.

The Error is easily fixed by specifying the feePayer before Signing the transaction.

  tx.recentBlockhash = (
    await program.provider.connection.getLatestBlockhash()
  ).blockhash;
  tx.feePayer = provider.wallet.publicKey // specify the feepayer before signing

  tx.sign(backend);
  const serializedTx = tx.serialize({ requireAllSignatures: false });