Sign the payer of the transaction through an API - solana

Question

What I would like to achieve is to make the user use the solana program for "free" and make the company pay for the transaction, what I have in mind is:
Extrapolate the transaction in the frontend
Send it to my backend server through an api
Use the wallet that I have on my BE to sing and set this wallet as payer
Send back the transaction
Sign the transaction with the user that is interacting with the FE
Send the transaction from the FE to the solana program.
Let's consider the hello world example https://github.com/solana-labs/example-helloworld
export async function sayHello(): Promise<void> {
console.log('Saying hello to', greetedPubkey.toBase58());
const instruction = new TransactionInstruction({
keys: [{pubkey: greetedPubkey, isSigner: false, isWritable: true}],
programId,
data: createSetInstruction()
});
console.log(instruction)
await sendAndConfirmTransaction(
connection,
new Transaction().add(instruction),
[payer],
);
}
I guess that in some way I could extrapolate the transaction before the sendAndConfirmTransaction
How can I achieve that and the sign it with my Backend wallet?
Update
In order to manage this problem, I started developing this service: cowsigner.com

Answer 1

You have the entire flow correct, so what you would do is:
const transaction = new Transaction(... your instructions here ...);
const wireTransaction = transaction.serialize();
// send wireTransaction to backend to be signed
// on backend:
const latestBlockhash = await connection.getLatestBlockhash();
transaction.lastValidBlockHeight = latestBlockhash.lastValidBlockHeight;
transaction.recentBlockhash = latestBlockhash.blockhash;
transaction.feePayer = backendKey.publicKey;
transaction.partialSign(backendKey);
const wireTransactionToSendBack = transaction.serialize();
// send back wireTransactionToSendBack to frontend to be signed by user
// back on frontend:
transaction.partialSign(userKey); // or use a wallet adapter, most likely
const finalWireTransaction = transaction.serialize();
const signature = await connection.sendRawTransaction(finalWireTransaction);