So, with 3 master public keys, and any three private keys each corresponding to the different master public keys, you can derive all the master private keys you're missing to be able to spend from the wallet.
If you have the 3 master public keys, then you would only need two private keys (not three) to be able to spend from a 2-of-3 wallet, as long as the two private keys both correspond to different master public keys. 
Topic: Multisig (Read 272 times)
Let's consider an alternate scenario where you have all the master public keys but you are missing two or three master private keys and this can't spend from the wallet.
If your derivation path was not hardened (and Electrum wallets are unhardened up to a point[1]), and you have an address' private key in addition to a master public key, it is possible to derive its corresponding master private key using an obscure BIP32 feature.
[1]: Electrum derives an extended key at m/84'/0'/0' as explained in the linked post, which is used to derive the addresses that it displays. That other derivation path is entirely unhardened and so it is possible to retrieve this extended key to get the master private key.
So, with 3 master public keys, and any three private keys each corresponding to the different master public keys, you can derive all the master private keys you're missing to be able to spend from the wallet.
If your derivation path was not hardened (and Electrum wallets are unhardened up to a point[1]), and you have an address' private key in addition to a master public key, it is possible to derive its corresponding master private key using an obscure BIP32 feature.
[1]: Electrum derives an extended key at m/84'/0'/0' as explained in the linked post, which is used to derive the addresses that it displays. That other derivation path is entirely unhardened and so it is possible to retrieve this extended key to get the master private key.
So, with 3 master public keys, and any three private keys each corresponding to the different master public keys, you can derive all the master private keys you're missing to be able to spend from the wallet.
To recreate a 2-of-3 wallet in Electrum... you absolutely require 3 master keys... Electrum will not allow you to use individual private/public keys to create a multisig.
Electrum will only create a multisig from seeds or master keys... To be able to recreate and spend from a 2-of-3 multisig wallet using Electrum you need, at a minimum:
- 2 seeds or 2 master private keys
- 1 master public key
That would allow you to recreate the wallet and be able to spend from it... as the wallet file would contain the necessary private keys to be able to sign transactions.
If you also had 3 seeds or 3 master private keys (or some combination that resulted in having 3 seeds and master private keys in total) then you would also be able to recreate the wallet file.
Electrum will only create a multisig from seeds or master keys... To be able to recreate and spend from a 2-of-3 multisig wallet using Electrum you need, at a minimum:
- 2 seeds or 2 master private keys
- 1 master public key
That would allow you to recreate the wallet and be able to spend from it... as the wallet file would contain the necessary private keys to be able to sign transactions.
If you also had 3 seeds or 3 master private keys (or some combination that resulted in having 3 seeds and master private keys in total) then you would also be able to recreate the wallet file.
What if i lost 1 xpubs. can i get acess to the wallet even if i have 2 private keys ? can i get the last xpubs that i lost?
No. There is no way to determine what the missing xpub is... if you only have 2 xprv's (or 2 seeds), then without either the 3rd seed, xprv or xpub, there is no way you can recreate the wallet in Electrum. if i use electrum to check my 3 public master keys i see that they all start with zPub. Is this an xPub ( master public key) and everything that we talked about ?
Yes, the zpub is your master public key. The zpub is essentially the segwit (P2WSH in the case of this multi-sig wallet) version of the xpub, with the xpub being for legacy wallets (P2SH in the case of multi-sig). 
if i use electrum to check my 3 public master keys i see that they all start with zPub. Is this an xPub ( master public key) and everything that we talked about ?
Yes. That is the bech32 standard (bc1). It'll be wrong to generalize them as xPub as there are many variations of the master public key. 
if i use electrum to check my 3 public master keys i see that they all start with zPub. Is this an xPub ( master public key) and everything that we talked about ?
This is one of my questions. I am assuming one master public key is lost. And without the 3 master keys of a 2 of 3 multisig i think i cant recover the wallet even with the 3 private keys with me.
3 private keys to an individual address in the multisig wallet would only allow you to spend funds on that individual address. They would not allow you to spend funds on any other address in the multisig wallet. If you have the 3 master private keys, then you can recover the wallet.And a scenario where someone only has 2 private keys and 2 master public keys. Can he get acess on a 2of3 multi sig?
If you the 2 xpubs are derived from the 2 xprvs then no. If one of the xpubs is from the missing xprv, then yes.someone here also said that you can get all master public keys from a transaction from the multi sig wallet. is that true?
I don't think so. You will be able to extract the three public key to that individual address if it has made a transaction from the redeemScript. You will not be able to extract the xpubs.someone here also said that you can get all master public keys from a transaction from the multi sig wallet. is that true?
You can get a redeem script of the multisig address if you're able to get a transaction for which the inputs are from the multisig address. This gives you all of the possible public keys that are in the redeem script. Master public keys are not located in the redeem script or at least I've never seen any redeem scripts like that. For wallets that uses xpubs to generate multiple multisigs in a HD manner, each of the redeem script contains the public keys of the addresses that are generated deterministically from the xpub and at the specific indexes for example the first address contains the public keys from the first address generated by the 3 xpubs, and so on.Let me know if you need an example of a redeem script from a transaction.
But if I'm not mistaken, only the xpubs of the wallets that signed the transaction can be extracted from the TXID. So, if two private keys sign a 2-of-3 muli-sig wallet's a transaction only one of those two keys' xpubs can be extracted. Furthermore, I believe one needs to know the remaining xpubs to extract the unknown xpub.
No you can't. If you're using xpubs to generate those multisigs, the wallet actually uses the public keys from each of the individual xpubs and the xpubs are never defined.Redeem scripts are not revealed to others unless in a transaction. Your addresses are hash of the redeem script.
someone here also said that you can get all master public keys from a transaction from the multi sig wallet. is that true?
I believe almost anything LoyceV says, I've never encountered an occasion to doubt him. I've never tried to do such a thing myself, so I don't know how it's done. But if I'm not mistaken, only the xpubs of the wallets that signed the transaction can be extracted from the TXID. So, if two private keys sign a 2-of-3 muli-sig wallet's a transaction only one of those two keys' xpubs can be extracted. Furthermore, I believe one needs to know the remaining xpubs (the other two, in your case) to extract the unknown xpub.
And a scenario where someone only has 2 private keys and 2 master public keys. Can he get acess on a 2of3 multi sig?
Yes, but only if one of those public keys is not the derivative of either of the two private keys in his possession. To steal the funds from a 2-of-3 multi-sig wallet the hacker would have to have at least two private keys, and the public key of the third wallet.
someone here also said that you can get all master public keys from a transaction from the multi sig wallet. is that true?
And a scenario where someone only has 2 private keys and 2 master public keys. Can he get acess on a 2of3 multi sig?
Yes, but only if one of those public keys is not the derivative of either of the two private keys in his possession. To steal the funds from a 2-of-3 multi-sig wallet the hacker would have to have at least two private keys, and the public key of the third wallet.
This is one of my questions. I am assuming one master public key is lost. And without the 3 master keys of a 2 of 3 multisig i think i cant recover the wallet even with the 3 private keys with me.
If you have all three private keys, you can restore the wallet by creating a new multi-sig wallet with those private keys. You'll be able extract the public keys in the process. The public key's are derived from the private keys.
And a scenario where someone only has 2 private keys and 2 master public keys. Can he get acess on a 2of3 multi sig?
This is one of my questions. I am assuming one master public key is lost. And without the 3 master keys of a 2 of 3 multisig i think i cant recover the wallet even with the 3 private keys with me.
If you have all three private keys, you can restore the wallet by creating a new multi-sig wallet with those private keys. You'll be able extract the public keys in the process. The public keys are derived from the private keys.
What if i lost 1 xpubs. can i get acess to the wallet even if i have 2 private keys ? can i get the last xpubs that i lost?
As far as I know, you could use an old transaction from the same multisig address to get what you need (but I couldn't tell you how).Private keys are used to access multisig wallets, the private keys can regenerate the public keys, so that is not an issue if you have the private keys.
For a 2 of 3 multisig, you'll need 2 private keys and one public key. If you don't have that public key, you can't move funds.This is one of my questions. I am assuming one master public key is lost. And without the 3 master keys of a 2 of 3 multisig i think i cant recover the wallet even with the 3 private keys with me.
What if i lost 1 xpubs. can i get acess to the wallet even if i have 2 private keys ? can i get the last xpubs that i lost?
As far as I know, you could use an old transaction from the same multisig address to get what you need (but I couldn't tell you how).Private keys are used to access multisig wallets, the private keys can regenerate the public keys, so that is not an issue if you have the private keys.
For a 2 of 3 multisig, you'll need 2 private keys and one public key. If you don't have that public key, you can't move funds. 
If a hacker has access to your master public key the only thing they can do is see your public addresses. Your privacy will be compromised, but your funds will remain secure. If your seed phrase or your master private key for a multi-sig wallet were compromised, the hacker still won't be able to sign transactions unless he has access to remaining quantity of xpriv keys needed.
You are right, but it does not necessarily mean the hacker should have access to all the private key before the wallet will be compromised, it depends on the M of N multisig wallet that was created. Like in the 2 of 3 multisig wallet I explained above, the hacker only needs to know 2 out of the 3 private key to compromised the wallet, but in 2 of 2 multisig wallet, the hack needs all (the two) private key to compromise the wallet.
Its a 2-3 multi sig. What i mean by xpub is the master public key, not the private key.
If a hacker has access to your master public key the only thing they can do is see your public addresses. Your privacy will be compromised, but your funds will remain secure. If your seed phrase or your master private key for a multi-sig wallet were compromised, the hacker still won't be able to sign transactions unless he has access to remaining quantity of xpriv keys needed.
What if i lost 1 xpubs. can i get acess to the wallet even if i have 2 private keys ? can i get the last xpubs that i lost?
You mean if you lost one private key? It depends, if it is 2 of 2 multisig wallet, you will not be able to access the fund again if you lost one private key. So, it depends on the numbers of cosigners. If you do not lost more than or exactly the number of co-signer private key, you will still be able to access your funds.
But, if you lost (public key), you can still use private key to regenerate public key. Private key is the one needed to not be lost.
What if i lost 1 xpubs. can i get acess to the wallet even if i have 2 private keys ? can i get the last xpubs that i lost?
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