Easy way to specify redeemScript while signing transaction

[motatoes]

I ran into a problem when I was trying to use coinbin within a multisig spending. I've documented what I was tyring to do here. In summary, I was trying to generate a transaction in the core to spend out funds from a 2-of-2 multisig address. I wanted to partially sign the transaction on coinbin using 1 of the private keys, and then do the second signature in bitcoind. I kept getting the message error": "Script evaluated without error but finished with a false/empty top stack elementwhen I was tyring to get the final signature inbitocoind`.

After debugging this issue further I realised that bitcoind generates a transaction with an empty scriptSig for the inputs, and so coinbin signs it as if it were a normal pay to address transaction (I think). Unfortunately I was not able to solve my issue in the UI, but I did manage to solve it by manually modifying the transaction in JS as follows:

var rawtx2 = "01000000021a1f40dba3ab5c208b165d47bdcf2aadc517c4f2b90b781eab53dadcb35d61200100000000ffffffff05ec447a886d8711db0d5f4cbf1f61956cf227c82c5594ee195bb179e86faa910100000000ffffffff0138c70000000000001976a91472a2a94468e42593656cc9803c7a48c1863fc3e588ac00000000";


(function partialSignMultisig(window) {
	var coinjs = window.coinjs;
	var wifkey = "<WIFKEYHERE>";
	var script = rawtx2;
	var redeemScript = "5221020150ce6954f0cdcc2c5d57cf249f25eab0eed23e39549c4f06bcb42a37f308ce2102224aca2849ddad9c663c0bcfdac5ce3ca06f784ac8eca83404a4ea4ac61dbe7d52ae";

	var tx = coinjs.transaction();
  	var t = tx.deserialize(script);
        
        // Manually specify scripts for inputs ..
	for (var i = 0; i < t.ins.length; i++) {
		t.ins[i].script = coinjs.script(redeemScript);
	}

	var signed = t.sign(wifkey);
	console.log( signed );


})(window);

However, I think the UI responsible for signing should also allow us to specify the redeemScript to use during the signature .. In particular I think we can merge the transaction verification UI as a first step for the signature. So the user can decode the transaction as a first step, and optionally specify the private key for each input in the transaction, and also a redeemScript (?) if any. This is also similar to the API that bitcoind facilities as a second argument to the signrawtransaction endpoint. This issue is also related to #62

[dabura667]

Currently there is no specification of partially signed transactions, so Bitcoin Core uses one method, Electrum uses another, Coinb.in another, Copay another...

Someone should write a BIP to standardize serialization for partially signed / unsigned raw transactions...

[motatoes]

Maybe until standardization we should ensure that it works with the most popular client (core) ?

[dabura667]

we should ensure that it works with the most popular client (core) ?

Pull requests are welcome. Just be sure to keep backwards compatibility in case someone is sitting on a partially signed transaction waiting to finish it in 5 years.

[motatoes]

@dabura667 How should I do it in the UI side ? Should it be like adding a field "redeem Script" and setting all the input scripts to that value before signing?