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A transaction is a transfer of Bitcoin value that is broadcast to the network and collected into blocks. A transaction typically references previous transaction outputs as new transaction inputs and dedicates all input Bitcoin values to new outputs.
Transactions are not encrypted, so it is possible to browse and view every transaction ever collected into a block. Once transactions are buried under enough confirmations they can be considered irreversible.
Standard transaction outputs nominate addresses , and the redemption of any future inputs requires a relevant signature. All transactions are visible in the block chain , and can be viewed with a hex editor. A block chain browser is a site where every transaction included within the block chain can be viewed in human-readable terms. This is useful for seeing the technical details of transactions in action and for verifying payments. The input in this transaction imports 50 BTC from output 0 in transaction f5d Then the output sends 50 BTC to a Bitcoin address expressed here in hexadecimal When the recipient wants to spend this money, he will reference output 0 of this transaction in an input of his own transaction.
An input is a reference to an output from a previous transaction. Multiple inputs are often listed in a transaction. All of the new transaction's input values that is, the total coin value of the previous outputs referenced by the new transaction's inputs are added up, and the total less any transaction fee is completely used by the outputs of the new transaction.
Previous tx is a hash of a previous transaction. Index is the specific output in the referenced transaction. ScriptSig is the first half of a script discussed in more detail later. The script contains two components, a signature and a public key.
The public key must match the hash given in the script of the redeemed output. The public key is used to verify the redeemers signature, which is the second component. More precisely, the second component is an ECDSA signature over a hash of a simplified version of the transaction. It, combined with the public key, proves the transaction was created by the real owner of the address in question.
Various flags define how the transaction is simplified and can be used to create different types of payment.
An output contains instructions for sending bitcoins. ScriptPubKey is the second half of a script discussed later.
There can be more than one output, and they share the combined value of the inputs. Because each output from one transaction can only ever be referenced once by an input of a subsequent transaction, the entire combined input value needs to be sent in an output if you don't want to lose it.
Any input bitcoins not redeemed in an output is considered a transaction fee ; whoever generates the block will get it. To verify that inputs are authorized to collect the values of referenced outputs, Bitcoin uses a custom Forth-like scripting system.
The input's scriptSig and the referenced output's scriptPubKey are evaluated in that order , with scriptPubKey using the values left on the stack by scriptSig. The input is authorized if scriptPubKey returns true.
Through the scripting system, the sender can create very complex conditions that people have to meet in order to claim the output's value. For example, it's possible to create an output that can be claimed by anyone without any authorization.
It's also possible to require that an input be signed by ten different keys, or be redeemable with a password instead of a key. It is possible to design more complex types of transactions, and link them together into cryptographically enforced agreements.
These are known as Contracts. A Bitcoin address is only a hash, so the sender can't provide a full public key in scriptPubKey. When redeeming coins that have been sent to a Bitcoin address, the recipient provides both the signature and the public key.
The script verifies that the provided public key does hash to the hash in scriptPubKey, and then it also checks the signature against the public key. P2SH addresses were created with the motivation of moving "the responsibility for supplying the conditions to redeem a transaction from the sender of the funds to the redeemer. They allow the sender to fund an arbitrary transaction, no matter how complicated, using a byte hash" 1. Pay-to-Pubkey-hash addresses are similarly a byte hash of the public key.
Pay-to-script-hash provides a means for complicated transactions, unlike the Pay-to-pubkey-hash, which has a specific definition for scriptPubKey, and scriptSig. The specification places no limitations on the script, and hence absolutely any contract can be funded using these addresses. The scriptPubKey in the funding transaction is script which ensures that the script supplied in the redeeming transaction hashes to the script used to create the address. In the scriptSig above, 'signatures' refers to any script which is sufficient to satisfy the following serialized script.
Generations have a single input, and this input has a " coinbase " parameter instead of a scriptSig. The data in "coinbase" can be anything; it isn't used. Bitcoin puts the current compact-format target and the arbitrary-precision "extraNonce" number there, which increments every time the Nonce field in the block header overflows.
Outputs can be anything, but Bitcoin creates one exactly like an IP address transaction. The extranonce contributes to enlarge the domain for the proof of work function. Miners can easily modify nonce 4byte , timestamp and extranonce 2 to bytes. The input sufficiently describes where and how to get the bitcoin amout to be redeemed.
If it is the only input of the first transaction of a block, it is called the generation transaction input and its content completely ignored. Historically the Previous Transaction hash is 0 and the Previous Txout-index is The output sets the conditions to release this bitcoin amount later.
The sum of the output values of the first transaction is the value of the mined bitcoins for the block plus possible transactions fees of the other transactions in the block. Retrieved from " https: Navigation menu Personal tools Create account Log in. Views Read View source View history. Sister projects Essays Source. This page was last edited on 28 March , at Content is available under Creative Commons Attribution 3. Privacy policy About Bitcoin Wiki Disclaimers.