Topic: Using the "same" bitcoin on multiple computers (Read 3771 times)

LOL i think maybe talk to mt. gox and the foundation about those mystery ghost blocks, wait someone will prolly come out with a new improved ghost block cracker sniffer and call it ghostbusters' miner NYC;)

maybe this is what makes up many of these early created "mystery blocks" can we come out with a dead mystery bitcoin sniffer? NYC;)

In the situation being discussed here, they go to the address they were sent to, and then they just stay there.  Since nobody has the private key for that address any longer, they'll simply never be spent again (unless in the future some mathematicians discover serious flaws in all three of the ECDSA, SHA-256, and RIPEMD-160 algorithms).

There are other ways to lose bitcoins whereby they simply vanish from existence, but that is rare, unlikely, and requires a miner to be running some software with a serious bug.

where do permanently lost bitcoins go? NYC;)

are we allowed to write some new code? what if we all do it at once maybe it's possible to double our btc coinage; basically a forward split to reward all current bitcoin enthusiasts NYC;)

You didn't specify, so I'm assuming you are asking about the Bitcoin-Qt wallet. Copy and paste the entire contents of the "Bitcoin" directory from one computer to the other.
If you are asking about Electrum, MultiBit, or Armory, then the process might be different.


Yes.  Keep in mind that you should then stop using the wallet on the old computer.  Attempting to use the same Bitcoin-Qt wallet on multiple computers is a recipe for disaster and carries a high risk of permanent loss of bitcoins.


This is not true if you don't want to "have to reload again all blocks from the internet".  To avoid reloading all the blocks, you also have to copy the entire blockchain.


Correct.  That is the Windows directory you would need to copy.


Depending on the version of Bitcoin-Qt that you are copying, you may need to copy more than just 2 files.


This can be done with the Armory wallet.  Bitcoin-Qt is not designed to be used in this way.


Sending any bitcoins from both wallets, it doesn't have to be the "same" bitcoin.  There are a variety of potential issues since Bitcoin-Qt doesn't let you choose which bitcoins you are sending.  The Bitcoin-Qt wallet is not designed to stay synchronized with a second copy of the same wallet.  One of the biggest issues you are likely to run into is caused by the way the Bitcoin-Qt wallet handles the "change" from transactions.  After you send enough transactions, you'll start to see extra bitcoins vanish out of the copy of the wallet that didn't send the transaction.  If you send transactions from both copies of the wallet, you'll eventually see the two wallets showing completely different balances.  Trying to get them re-synchronized without permanently losing some of those bitcoins will be a complicated process that will carry a risk of permanent loss if you make a mistake.


If you simply create a new address in the wallet that the thief has a copy of, he'll still be able to steal them.  You need to create a new address in a completely new wallet.  Then send the entire balance from the old wallet to the new wallet.  If you can accomplish this before the thief has transferred the bitcoins to his own wallet, then they will be secure.  Of course, this all assumes that you have some way of knowing that a thief has a copy of your wallet before he transfers the bitcoins.


No.  The only thing you need is the private keys.  The public keys (and addresses) can be calculated from the private keys. It is important to be absolutely certain that you have the correct private keys though.  Bitcoin-Qt stores many of your bitcoins in addresses that it doesn't tell you about.  If you only have the private keys of the addresses that are in the "Receive Coins" section of your wallet GUI, then you probably don't have everything you need.  The easiest way to be certain that you have all the necessary private keys is to create a backup of the wallet.dat file on a regular basis (perhaps every 25 transactions or so) and store multiple backups (perhaps the 3 most recent backups).


It isn't so much a "bug" as it is using the wallet in a way that it was not intended to be used.  It was not designed to remain synchronized with multiple copies of the same wallet.dat file.  You are welcome to try it out if you like.  It will most likely appear to be working just fine for a while.  After you send 100 transactions or so, you'll see the risk of bitcoin loss jump significantly.

Here's a scenario where you can potentially run into double-spend issues:

• Create two copies of a brand new wallet.dat file running one on each of two computers (copy_A and copy_B)
• Send a single transaction of 2 BTC to a single address that exists in both copies of the wallet
• Wait for the transaction to get 6 confirmations
• Put copy_A into an offline state (disconnect the network cable, turn off the computer, or simply shut down the Bitcoin-Qt software)
• Keep copy_B online
• Wait a few days so that the blockchain in copy_A falls behind on synchronization
• Send a 0.5 BTC transaction from copy_B to some other address that is not part of these two wallets
• See that the balance on copy_B has dropped to 1.5 BTC
• Bring copy_A online
• See that the balance on copy_A is still 2 BTC
• Send a transaction from copy_A  to some other address that is not part of these two wallets before copy_A catches up in synchronization

This process will result in a double-spend problem in copy_A.  It will reduce the balance in copy_A, and will show the transaction in the transaction list, but it will never confirm.


FUD?  You are aware that Bitcoin-Qt is not designed for this and that there is a significant risk of permanent loss of bitcoins if you do so, right?

In short, once you have spent bitcoins from one wallet, you no longer have 2 copies of the same wallet. Instead, you have 2 different wallets that happen to share some keys.

The biggest problem arises after 100 addresses have been used.  Since Bitcoin-Qt generates random private keys instead of using a deterministic algorithm, the change from the transaction will be sent to an address that only exists in one copy of the wallet.  As such you no longer have "2 copies" of the private key.  You only have one.  Now if you are sending transactions from both wallets, you'll end up with some bitcoins only associated with addresses in one copy, some bitcoins only associated with addresses in the other copy, and some bitcoins associated with addresses in both copies.  The average non-technical user who has been happily using their two copies of the wallet without seeing any issues suddenly notices that the two copies of the wallet no longer have the same amount of bitcoins, but has no understanding of how or why this has happened.  What they do next to attempt to resolve this issue on their own carries risk of losing the private keys from one of the copies of the wallet (and as such all the bitcoins associated with private keys that only belonged to that "copy").

This method would work, and as you point out is an alternative to using the pywallet tool if you've encountered a double-spend problem.  Unfortunately, this method requires significantly more technical expertise.  It is important to be aware that when J35st3r says "export the private keys from both wallets to a text file", it is implied that you know how to determine the full list of all private keys stored by each wallet.

But that does not solve the double-spend problem, leaving some coins in limbo.

My solution would be to export the private keys from both wallets to a text file. Do a sort/merge so only one copy of each key exists, then import them into a freshly created wallet. This will then synchronise with the full balance and any double-spends from the old wallets will just die out as the network drops them from the memory pool.

I guess the best way to resolve this would be making a new wallet->sending to it all bitcoins from the first wallet->waiting for 6 six confirmations->opening the second wallet.dat and sending the remaining bitcoins to the new wallet

I guess the best way to resolve this would be making a new wallet->sending to it all bitcoins from the first wallet->waiting for 6 six confirmations->opening the second wallet.dat and sending the remaining bitcoins to the new wallet

If you don't wait for complete synchronization, you won't lose bitcoins.  You'll just end up with a double spend that will never confirm.  Then you'll have to use a tool like pywallet to fix the wallet.

The real issue arises after you've sent about 100 transactions without replacing one copy of the wallet.dat with the other.    This problem is due to the way Bitcoin-Qt handles address generation and transaction change.  It won't matter if the blockchain is fully synchronized or not. At that time you'll have exhausted the pre-generated address pool, and you'll start to see extra bitcoins disappear out of the wallets.  The problem can be resolved safely if you are careful and know what you are doing.

The real issue arises after you've sent about 100 transactions without replacing one copy of the wallet.dat with the other.    This problem is due to the way Bitcoin-Qt handles address generation and transaction change.  It won't matter if the blockchain is fully synchronized or not. At that time you'll have exhausted the pre-generated address pool, and you'll start to see extra bitcoins disappear out of the wallets.  The problem can be resolved safely if you are careful and know what you are doing.

So if you have 2 copies of the same wallet.dat (at least; the same when originally copied), it is possible to lose Bitcoins this way. Wow, that's news to me. I've been using 2 copies of my wallet.dat for a long time now and never noticed any issues.

In that scenario would an older backup of wallet.dat regain those lost Bitcoins by simply letting it synchronize? Or does the other copy receive the same transaction and lose the Bitcoins as well?

Is it always perfectly safe to use 2 copies if you wait for complete synchronization before sending any Bitcoins to anyone? (This is what I've done till now)

The more I learn about bitcoin the more concerned I am about the potential. The average person can barely handle fiat money, but bitcoins will be a disaster in the hands of most people.

FUD?  You are aware that Bitcoin-Qt is not designed for this and that there is a significant risk of permanent loss of bitcoins if you do so, right?