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Many merchants wait for at least 6 confirmations of a transaction meaning that six subsequent blocks of transactions were added to the blockchain after the transaction in question. At this point, the merchant can safely assume that the transaction is valid. There remain other vulnerabilities in this system which could allow double-spend attacks to take place.

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If an attacker were somehow able to get control of this much computational power, they could reverse transactions and create a separate, private blockchain. However, the rapid growth of bitcoin has virtually insured that this type of attack is impossible.


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Now let's get a little more technical. The way that users detect tampering such as an attempt to double-spend in practice is through hashes , long strings of numbers that serve as proof of work PoW. Put a given set of data through a hash function bitcoin uses SHA , and it will only ever generate one hash. Due to the "avalanche effect," however, even a tiny change to any portion of the original data will result in a totally unrecognizable hash. Whatever the size of the original data set, the hash generated by a given function will be the same length.

The hash is a one-way function: it cannot be used to obtain the original data, only to check that the data that generated the hash matches the original data. Generating just any hash for a set of bitcoin transactions would be trivial for a modern computer, so in order to turn the process into "work," the bitcoin network sets a certain level of "difficulty. Setting difficulty is accomplished by establishing a "target" for the hash : the lower the target, the smaller the set of valid hashes, and the harder it is to generate one.

In practice, this means a hash that starts with a long string of zeros: the hash for block , for example, is ddefdbb1bd75e8d78ff2e8d. That block contains 2, transactions involving just over 1, bitcoin, as well as the header of the previous block. If a user changed one transaction amount by 0. Since a given set of data can only generate one hash, how do miners make sure they generate a hash below the target?

They alter the input by adding an integer, called a nonce "number used once". Once a valid hash is found, it is broadcast to the network, and the block is added to the blockchain. Mining is a competitive process, but it is more of a lottery than a race. On average, someone will generate acceptable proof of work every ten minutes, but who it will be is anyone's guess.

Miners pool together to increase their chances of mining blocks, which generates transaction fees and, for a limited time, a reward of newly-created bitcoins. Proof of work makes it extremely difficult to alter any aspect of the blockchain, since such an alteration would require re-mining all subsequent blocks. It also makes it difficult for a user or pool of users to monopolize the network's computing power, since the machinery and power required to complete the hash functions are expensive.


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Why is my transaction unconfirmed/stuck for hours/days? : Coinomi Support

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Waiting for the network to "forget" about the transaction

Dealing With Double Spending. Key Takeaways A technical issue that arises with the notion of a digital currency is the ability for somebody to duplicate the digital money and spend it simultaneously at two or more places. This 'double-spend' problem is prevented in blockchain-based cryptocurrencies such as Bitcoin by using a consensus mechanism known as proof-of-work PoW. This PoW is carried out by a decentralized network of 'miners' who not only secure the fidelity of the past transactions on the blockchain's ledger but also detect and prevent double-spending.

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What is a "Stuck" transaction? How are they caused?

The offers that appear in this table are from partnerships from which Investopedia receives compensation. Related Articles. Bitcoin Bitcoin vs. Bitcoin Cash: What Is the Difference? When potential loss due to double spending as nominal, as with very inexpensive or non-fungible items, people may choose not to wait for a transaction to be confirmed, and complete the exchange as soon as it is seen on the network.

Most exchanges and other merchants who bear the risk from double spending require 6 or more blocks. There is nothing special about the default, often-cited figure of 6 blocks. Freshly-mined coins cannot be spent for blocks. It is advisable to wait some additional time for a better chance that the transaction will be propagated by all nodes. Some older bitcoin clients won't show generated coins as confirmed until they are blocks deep. Note that in the reality of bitcoin mining today, more than 6 confirmations are required. Some mining enterprises may hide their hash power across several mining pools.

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Also mining ASICs can be temporarily overclocked to increase their hash power. This is less power-efficient but could be used for a brief burst of hashrate. See also: Irreversible Transactions. Each additional confirmation is a new block being found and added to the end of the blockchain.

Miners create blocks by solving the proof of work for their proposed block. The block interval has an average of 10 minutes but not every block interval is exactly 10 minutes. It follows a statistical process known as a poisson process , where random events happen with the same probability in each time interval. Another way of expressing this is that the mining process has no memory, at every second a block has the same chance of being found.

Poisson processes are well-understood but can be unintuative.