Where to find private key to sign transaction?

[ranochigo]

I think the best way to identify whichever key you're looking for is to find the redeem script.

Copy the string from the script column in your inputs and it should be a format like 0047....ae. Remove the zeros and check the number after that, if it is 47, remove 142 characters, 48, remove 144 characters or if it is 49, remove 146 characters. So, you'll be removing ... 0047(142/144/146 characters) and the remainder should be 52....53ae, if not then try again. Paste the rest of the string back into coinb.in/#Verify and it should display the redeem script and the addresses associated with that multisig.
 
Unfortunately, I can't recall any site that displays the redeemscript from a raw transaction so this is actually quite an inelegant solution.

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I believe that decoding your redeem script would help you to see the public keys required for a valid signature and possibly aid you in finding the correct keys to use. Let me know if you don't understand anything about this.

If done correctly, this should be shown on your verify tab:

[o_e_l_e_o]

As I said above. The transaction is coming from a multi-sig address, and has been signed by the vendor with one of the three private keys. If you are unwilling to reveal the name of the vendor or the details of the raw transaction, then as to where the other two keys are we can only guess.

You could try contacting the vendor and asking them if you are supposed to have one or both of the other keys and what format they would be in.
Check your account with the vendor, looking for any mention of a private key or seed phrase which you could export.
Check your the email account you used to sign up, and see if they have emailed you a private key or seed phrase.
Check all the files or back ups you made around the time of opening the account.

[HCP]

... but yes it uses multi sig system. upon registering on the site I've entered my refund address and public key.

What was the refund address and public key that you entered when you registered on this site?

so pretty much i'm stuck with the same issue as i don't have the private key. Where and how should I have obtained this private key in this situation?

The private key, will likely be the one that is paired with the public key that you entered when you registered on the site. Unfortunately for you, there is no way to go from public key -> private key (otherwise the entire system would be fundamentally broken! )

At best, we can identify what the address is (convert the public key to address, see below), and then hopefully you will be able to find that address in your Electrum wallet and will be able to export the private key from it.




You can convert the public key to address really easily here: https://gobittest.appspot.com/Address

Simply copy/paste the public key into Box labelled: "1 - Public ECDSA Key" and click "send":


It'll most likely give you the "Invalid Public Key Length" error... but you can ignore that (the site was designed for older uncompressed public keys that start with 04, but still works with compressed public keys that start with 02 or 03)... then simply scroll down to Box labelled: "9 - Base58 encoding of 8"... and it will show the address associated with that public key: