Partially signed transactions should use OP_0 as placeholder

[ysobolev]

I believe many multisig wallets (CoinVaut, BitGo) use the convention that OP_0 is used as a placeholder for missing signatures. I am not sure if it is a standard, but it does appear to be used in many places. Can ScriptMultisig._dummy_signature please be changed to return "\x00" instead? Thanks.

[richardkiss]

I agree that there should be a standard placeholder, but my gut feeling has been to put a 0000...0 signature the same length of a standard signature. This way, a partially signed transaction is exactly the same length as the final transaction, which makes it a bit easier to calculate the required transaction fees. I think this is better than OP_0 even though it doesn't match the defacto "standard" that others have adopted. What do you think?

[ysobolev]

Well, you have an excellent point about fees. By the way, right now, the signature is not all zeros. Anyway, if you want to try to champion another standard, I support that. I agree, OP_0 is not exactly the best placeholder. However, if the majority of users settle on OP_0, that is what you should go with.

[sserrano44]

I had this same issue trying the cryptocorp api and had to monkey patch pycoin to make it work

[richardkiss]

Sebastian, can I take a look at your patch?

[ysobolev]

If using 0.53 from GitHub, then the following code works well:
ScriptMultisig._dummy_signature = lambda x, y: "\x00"
If using 0.52, there appears to be a bug and ScriptMultisig.solve needs a rewrite.

[sserrano44]

We endup fixing the input and redeem script

from pycoin.ecdsa import generator_secp256k1
from pycoin.serialize import b2h, stream_to_bytes
from pycoin.key.BIP32Node import BIP32Node
from pycoin.tx.pay_to import ScriptMultisig
from pycoin.tx.script.tools import *
from pycoin.tx.script import der

def dummy_signature(sig_type):
    order = generator_secp256k1.order()
    r, s = order - 1, order // 2
    return der.sigencode_der(r, s) + bytes_from_int(sig_type)

def fix_input_script(inp, redeem_script):
    """replace dummy signatures with OP_0 and add redeem script for digitaloracle compatibility"""
    dummy = b2h(dummy_signature(1))
    ops1 = []
    for op in opcode_list(inp.script):
        if op == dummy:
            op = 'OP_0'
        ops1.append(op)
    # FIXME hack to add redeem script omitted by pycoin
    ops1.append(b2h(redeem_script))
    inp.script = compile(' '.join(ops1))

[richardkiss]

As of d09b5e0, you should be able to pass in
signature_placeholder=OP_0 as a kwarg to Tx.sign to use OP_0 as your placeholder.