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I have a technical question regarding the signrawtransaction function in Bitcoin core when signing a partially-signed transaction.

I have tested different scenarios and signrawtransaction will always sort the signatures in the correct way in the scriptSig, regardless of the order the transaction is signed one-by-one.

Is this achieved by analyzing the scriptSig of the partially-signed transaction when signing, and then verifying each signature in the partially-signed transactions scriptSig against all public keys in the redeemscript and finally ordering them correctly? Or is there a better way of doing this?

Unfortunately I don't understand how this is done by reading the C-code of bitcoin core.

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The signatures must be ordered the same way as public keys. Otherwize transaction is invalid. This is how OP_CHECKMULTISIG(VERIFY) works.

Of course, you may use the private keys for signing in any order, because signatures are independent.

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  • Thanks for your answer. I know that the order in the final scriptSig must regard the redeemScript. But does the Bitcoin Core signrawtransaction-function deserialize the partially-signed scriptSig and order the signatures accordingly? This is done by actually running the verify-signature algorithm on each provided signature and each possible public-key from the redeemScript?
    – Bjarne Magnussen
    Oct 6, 2017 at 10:00
  • E.g.: with a 2-of-3 multisig (A,B,C), we first sign the newly created raw transaction with public-key C. Now this partially-signed tx is signed with public key B. To place the signature of B in the partially signed scriptSig containing signature of C, signrawtransaction must first check to which public-key the already given signature belongs to know if the signature of B has to be placed before or after?
    – Bjarne Magnussen
    Oct 6, 2017 at 10:01
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    >>>...by actually running the verify-signature algorithm on each provided signature and each possible public-key from the redeemScript? <<< Yes. github.com/bitcoin/bitcoin/blob/master/src/script/sign.cpp#L228
    – amaclin
    Oct 6, 2017 at 11:30
  • In fact you can implement your own algorithm for signing multisig transactions. You may use some "placeholders" and replace them with signatures later. Format of raw unsigned transaction is not a consensus rule :)
    – amaclin
    Oct 6, 2017 at 11:37
  • Thanks! I am implementing the signing function in another program and want to keep compatibility with the way that Core is signing multisig. Your information has helped me a lot!
    – Bjarne Magnussen
    Oct 6, 2017 at 11:44

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