How do you commit to the previous output's amount when signing a segwit input with rust-bitcoin?

Question

I'm trying to sign a segwit transaction in rust-bitcoin. To start with, my transaction setup looks as follows:

let previous_output = OutPoint::new(Txid::from_hex("991ab2b13f6bc6c13002d79d5e9775626a5e7328e14cd16837d50d1cc637dc6a").unwrap(), 0);
let tx_input = TxIn {
    previous_output,
    script_sig: Default::default(),
    sequence: 0xffffffff,
    witness: Default::default()
};

let tx_output = TxOut {
    value: 4999995000,
    script_pubkey: output_details.script_pubkey() // don't worry about this
};

let mut transaction = Transaction {
    version: 2,
    lock_time: 0,
    input: vec![tx_input],
    output: vec![tx_output]
};

In order to sign the transaction (using Schnorr, by the way), I extract the sighash as follows:

let signature_hash = transaction.signature_hash(0, &output_details.script_pubkey(), 0); // is that really the most idiomatic way of passing a sighash flag in rust-bitcoin?
let message = Message::from_slice(&signature_hash.to_vec()).unwrap();

When I print the hex-serialized message, I get the following value: fa42b9b0f54972b31712b2efbc86db1bf78aa833bc9c969ea7860ef38a25fbaf.

However, in order to sign a segwit transaction, the previous output's amount must also be committed to, and I'm not seeing a way to do that. As far as I can tell, the sighash I should be signing is in fact 61746ec9baa14bbe82586c7f149926b1492871c67dc7fe21e7cfe2f2260a1405.

Finally, though less relevantly, if things were to work fine, here's how my signature process would conlude:

let signature = private_key.sign_schnorr(message);
let signature_vec = signature.as_ref().to_vec();
transaction.input[0].witness = Witness::from_vec(vec![signature_vec]);

Thanks in advance for helping me figure this out!

Answer 1

There is a bitcoin::util::sighash module which has all of the sighashing stuff. I'm not certain that it is the correct way, but it is definitely a way to get the result that you want.

It looks like you have to create a SighashCache object, which you can then use to get all different kinds of sighashes for the transaction.

In your case, this code might make it work:

let previous_output_as_tx_out = TxOut {
    value: 50_0000_0000,
    script_pubkey: output_details.script_pubkey()
};
let prevouts = vec![previous_output_as_tx_out];

let mut sighash_cache = bitcoin::util::sighash::SighashCache::new(&transaction);
let input_index = 0;

// taproot_key_spend_signature_hash is the crucial part
let signature_hash = sighash_cache.taproot_key_spend_signature_hash(input_index, &Prevouts::All(&prevouts), SchnorrSighashType::Default).unwrap();
let message = Message::from_slice(&signature_hash.to_vec()).unwrap();