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I was looking at bitcoin rich list and found binance cold wallet

For this transaction in particular https://www.blockchain.com/btc/tx/afb6523e4f5f060a076dcc2f8e5eb7a1aa58a3831456c62da946e2b5a41c24f0 whats going on under the hood? Why is the same address 8 times as input with different balances(?) number ?

Thanks

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Bitcoin uses a UTXO (unspent transaction output) model for transactions, see here for why. This means each time you spend a coin you create an output that is assigned to an address (or multiple outputs to different addresses), and those outputs must references previous unspent outputs to your address. You have to spend all of the coins from the utxo, so usually there is a change output that goes back to the original owner (usually a different address that they own, for privacy reasons).

Because each output of a particular transaction can only be spent once, the outputs of all transactions included in the block chain can be categorized as either Unspent Transaction Outputs (UTXOs) or spent transaction outputs. For a payment to be valid, it must only use UTXOs as inputs. - https://bitcoin.org/en/developer-guide#block-chain-overview

You are seeing many unspent outputs on the left that the address 16ftSEQ4ctQFDtVZiUBusQUjRrGhM3JYwe is using to add up to the values on the right that it wants to send.

screenshot

  • Thanks but why is the same input is used several times and not only one? I understand when its differents outputs being part of an input but in this case it looks like the address is used several times and im not sure what the number between () means? thanks – Marie90 Sep 14 '18 at 21:09
  • These are multiple different payments to the same address that are being gathered by this transaction. You can think of a bitcoin transaction as gathering inputs into a pile of bitcoins and then distributing the bitcoins in that pile into one or more outputs. – David Schwartz Sep 14 '18 at 21:49
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Bitcoin keeps track of coins by 'unspent output' (UTXO), not by 'address balance'. There is no such thing as an 'address balance' at the detailed technical level (though you can use the blockchain data to figure out what an 'address balance' might be. Most blockchain explorers display this info according to 'address balance', so it is easy to get confused at first).

When someone sends you some bitcoin, they will create a transaction output that specifies your address as the new owner of those coins. Think of this output (a newly formed UTXO) as a physical coin you have received.

Now, lets pretend you have received 8 transactions, so you have 8 physical coins in your pocket. Those coins may all exist at the same address, but in order to spend them, you must pull all 8 coins out of your pocket. Despite existing at the same address, the coins have not joined together into one coin, they are still individual coins (UTXOs), and thus must be individually specified in your transaction.

This is why that transaction makes a payment by using 8 different UTXOs that all exist at the same address. A transaction must specify each UTXO you want to spend, not each address' balance.

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