The block chain is collaboratively maintained by anonymous peers on the network, so Bitcoin requires that each block prove a significant amount of work was invested in its creation to ensure that untrustworthy peers who want to modify past blocks have to work harder than honest peers who only want to add new blocks to the block chain.Ĭhaining blocks together makes it impossible to modify transactions included in any block without modifying all subsequent blocks. For example, in the illustration above, each transaction spends 10,000 satoshis fewer than it receives from its combined inputs, effectively paying a 10,000 satoshi transaction fee. Ignoring coinbase transactions (described later), if the value of a transaction’s outputs exceed its inputs, the transaction will be rejected-but if the inputs exceed the value of the outputs, any difference in value may be claimed as a transaction fee by the Bitcoin miner who creates the block containing that transaction. For a payment to be valid, it must only use UTXOs as inputs. Outputs are tied to transaction identifiers (TXIDs), which are the hashes of signed transactions.īecause 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.
Any subsequent reference is a forbidden double spend-an attempt to spend the same satoshis twice. A single transaction can create multiple outputs, as would be the case when sending to multiple addresses, but each output of a particular transaction can only be used as an input once in the block chain.
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