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Author Topic: Can someone explain this thread which i just read  (Read 813 times)
ninjapug
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June 19, 2011, 07:41:01 PM
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http://forum.bitcoin.org/index.php?action=search2

I don't understand what causes the 8999 BTC to disappear for good?

if i have 5 btc now and i do a backup.
Then i sent 1 btc to myself, load the old backup, i will lose 4 btc?


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luxgladius
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June 19, 2011, 08:06:24 PM
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Your link doesn't work, but I'll give it a shot. A transaction basically says "I'm authorized to use the outputs of this other transaction because I know this key, and I choose to send it here." So let's say your wallet A has key A for Transaction 1 which has 10 Bitcoins, and you back it up. Now if you authorize 1 Bitcoin to be sent to your same address that key A has, using Transaction 1 as the funding source. The client has to make use of all the funds in a transaction. By default, the client will create a new key, Key B, to send the other 9 Bitcoins too, and those will be stored in your wallet. However, if you delete your wallet and restore the backup, it only knows about Key A, not Key B. Therefore, the 9 Bitcoins in change you sent are gone forever as there is no way to get that key back.

Hope that helps.
ninjapug
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June 20, 2011, 01:56:51 AM
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Sorry the correct link should be
http://forum.bitcoin.org/index.php?topic=782.0
ninjapug
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June 20, 2011, 02:02:12 AM
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Your link doesn't work, but I'll give it a shot. A transaction basically says "I'm authorized to use the outputs of this other transaction because I know this key, and I choose to send it here." So let's say your wallet A has key A for Transaction 1 which has 10 Bitcoins, and you back it up. Now if you authorize 1 Bitcoin to be sent to your same address that key A has, using Transaction 1 as the funding source. The client has to make use of all the funds in a transaction. By default, the client will create a new key, Key B, to send the other 9 Bitcoins too, and those will be stored in your wallet. However, if you delete your wallet and restore the backup, it only knows about Key A, not Key B. Therefore, the 9 Bitcoins in change you sent are gone forever as there is no way to get that key back.

Hope that helps.

I think i get it. It is so dangerous. OK lets say
If this time someone sent me 1 BTC after i backup 5BTC and then load back that wallet. Will i lose 4 BTC as well.
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June 20, 2011, 02:04:54 AM
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No, you would need to be the one sending for this problem to occur.
luxgladius
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June 20, 2011, 01:19:26 PM
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Right, nothing can deauthorize funds from an address but the person with the right key, which is to say you, hopefully. The client could change its default and have any change sent back to your address, but they don't do that right now because of privacy. Having money go back to the same address would  make it clear which part of a transaction is payment and which part is change.  Also that would tend to leave you with the same address over time, which makes it correlate more strongly to you as an identity. However, this is all somewhat theoretical. In reality you can usually see clearly from the amounts which part of a transaction is payment and which part is change anyway, so they probably should include an option in the program where you can have change sent back to an address of your choosing (that would have its key backed up), rather than creating a new one by default.
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