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How long does it take for a Bitcoin transaction to be confirmed, Coin Center

How long does it take for a Bitcoin transaction to be confirmed?

Stanford researcher Dr. Joseph Bonneau explains the distinction inbetween “confirmed” and “unconfirmed” Bitcoin transactions.

Frequently ter popular descriptions of Bitcoin and ter the user interfaces of wallet software, a distinction is made inbetween “confirmed” and “unconfirmed” transactions. What is the difference?

At a high level, a transaction is only confirmed when it is permanently included te the Bitcoin blockchain. The blockchain is a ledger of all transactions te the history of Bitcoin. It is append-only, meaning fresh gegevens can be added to the end of the ledger, but gegevens can never be eliminated once included. This ledger is necessary to prevent double-spending, which is a key technical challenge te designing any cryptocurrency.

How Bitcoins are Transferred

Recall that if Alice “owns” some quantity of bitcoins, this indeed means she knows one or more cryptographic keys which have bot designated spil the controller of those coins te a transaction on the ledger which transferred the coins to Alice. Te order to transfer the coins to another entity, Alice will use thesis keys to produce a digital signature on the statement “I would like to redeem (spend) this transaction and send the value to X, Y, Z…” where X, Y, and Z will be fresh cryptographic addresses indicating keys known by other individuals (or perhaps Alice herself).

Now, suppose Alice signs a statement on hier own pc telling she wants to transfer some coins to Bob but never sends the statement to Bob. Ter this case, clearly the coins have not bot transferred. This is toughly like a tree falling te the forest with nobody around to hear it. However, sending the signed statement only to Bob is not enough, because Alice could have signed a conflicting transaction telling she wants to transfer the coins to Carol which she only sends to Carol. If Bob and Carol both accepted thesis statements spil indicating that they have received the coins from Alice, then Alice will have effectively spent hier coins twice!

This is where the idea of a global ledger comes ter. If Alice wants to transfer hier coins to Bob, she vereiste publish hier statement authorizing the transfer to the blockchain. The miners who maintain the blockchain will only include this transaction if Alice has not yet transferred the coins to anybody else, so once Bob sees the transaction emerge ter the blockchain he can be certain that he is the fresh holder. Even if Alice zometeen attempts to produce a statement telling she transfered the coins to Carol, it will never be accepted into the blockchain because the transaction transferring to Bob wasgoed published very first.

Blockchain Forks

So it seems elementary: a transaction is “unconfirmed” once it has bot produced and cryptographically signed and “confirmed” once it has bot successfully included ter the blockchain. Unluckily, the blockchain does not offerande strong consistency, meaning that any gegevens included ter the blockchain is ensured to be included forever. For technical reasons, the blockchain offers a weaker property called eventual consistency, meaning that eventually all parties will agree on the blockchain up to a certain ever-increasing prefix.

Waterput another way, the blockchain is a series of n blocks (presently almost 400,000), and at any given time the most latest several blocks are not assured to be permanently included. It is possible for the blockchain to fork by having numerous potential (often veranderlijk) blocks which eis to be the last block ter the chain. Eventually one of thesis blocks will win and be permanently included, but it won’t always be instantly clear which block this is. When an evidently valid block is substituted by a rivaling block, this is called a blockchain reorganization and the substituted block is called anorphan block.

Given this wij might be tempted to say a transaction is “confirmed” once it has bot included ter a block which is not the very last block ter the blockchain. However, it is possible (albeit zonderling) for the last n blocks to be orphaned ter a reorganization. This is exponentially less likely to occur the larger n gets. It typically happens numerous times a day, for example, that a single block is orphaned, but has happened only a few dozen times te history for n inbetween Two and Four, and exactly once for ngreater than Four (a 24-block reorganization te March 2013 due to a technical glitch).

Acceptable Confirmation

Barring technical glitches, formal modeling of Bitcoin suggests that large reorganizations are exponentially unlikely, but possible. Therefore wij can never say with certainty that a transaction is “confirmed” because it is always possible that a transaction will evidently be included ter the blockchain but be substituted by a large reorganization.

Ter practice, the community has adopted 6 blocks spil a standard confirmation period. That is, once a transaction is included te a block ter the blockchain which is followed up by at least 6 extra blocks, the transaction is called “confirmed.” While this wasgoed chosen somewhat arbitrarily, it is a reasonably safe value te practice spil the only time this would have left users vulnerable to double-spending wasgoed the atypical March 2013 fork.

For very large transactions, coin owners might want to wait for a larger number of “block confirmations” and most wallet software now says more precisely that a transaction is “confirmed by n blocks” to enable users to determine for themselves if more confirmation is needed.

For relatively petite transactions (like buying coffee), users might be fine with a shorter confirmation period such spil one or even zero blocks. Even with only 1 confirmation (accepting a transaction once it is included ter the most latest block ter the chain) the risk of losing it to a reorganization is low (1-2%) and even then it will very likely be re-included after the reorganization occurs.

How Long Does 6 Blocks Take?

Assuming Alice is fine with the community standard of 6 blocks, how long will she have to wait? “One hour” is the common response but this is not fairly the entire story. Because blocks are found by a random process, there is no telling precisely how long it will take for 6 blocks to be found. On average, it takes about Ten minutes to find each block. The average block time can actually be slightly shorter or longer depending on if the total hash power of the Bitcoin network is growing or shrinking. Overlooking this detail tho’, this is why 6 confirmations take about 1 hour on average. However, the block-creation (or mining) process is random and each block may take much longer or shorter.

Conclusion

At a high level, a transaction is confirmed when it is permanently included ter the Bitcoin blockchain. The significant takeaway tho’ is that there is no absolute notion of “permanently included” and the community simply uses a reasonably safe policy of considering transactions confirmed when they are “included with very high probability.” The time it takes for this to toebijten is fairly variable-sometimes confirmation may be ems of minutes and sometimes it may take overheen two hours, but on average it will take about an hour.

Dr. Joseph Bonneau is a researcher at Stanford University and a technology fellow at the Electronic Frontier Foundation. He has instructed several university courses on cryptocurrency technology and is presently writing a textbook on the subject.

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