Key Takeaways
Cryptocurrency transactions are recorded on a public ledger (blockchain), making them transparent and permanent.
Transactions are validated by a decentralised network of computers using consensus mechanisms, primarily Proof of Work and Proof of Stake.
In September 2022, Ethereum switched from Proof of Work to Proof of Stake, a shift known as The Merge, significantly reducing the network's energy consumption.
Before a transaction is confirmed, it sits in the mempool, where validators and miners select which transactions to process next, typically prioritising higher fees.
Consensus mechanisms prevent fraud and double-spending without relying on a central authority such as a bank.
Introduction
Bitcoin and most other cryptocurrencies work differently from traditional money because they don't rely on banks or central authorities to check and approve transactions. Instead, the verifications are done through a public ledger, the blockchain.
You can think of a blockchain as a giant public notebook that everyone can see, but no one can alter without broad network consensus. This article explains how crypto transactions are checked and verified using consensus mechanisms like Proof of Work and Proof of Stake.
What Is a Cryptocurrency Transaction?
When you send cryptocurrency to someone, you create a transaction. This transaction records the sender and recipient wallet addresses, how many coins are involved, and when it happened. Once submitted, the transaction is broadcast to the network so it can be checked and confirmed.
How Are Blockchain Transactions Verified?
Sharing and checking
When you create a transaction, you use your wallet keys to generate a digital signature. Your transaction data and signature are then broadcast to the network of computers (nodes) that make up the blockchain. These nodes check the validity of your signature, that the required funds are available, transaction details are correct, and that the transaction follows the network’s rules.
The mempool
Once a transaction passes initial checks, it enters the mempool (short for memory pool). This is a waiting area where valid but unconfirmed transactions queue to be included in the next block. Validators and miners select which transactions to process, typically prioritising those with higher transaction fees. During periods of high network activity, the mempool can become congested, causing delays and pushing fees up.
How the network agrees
The network doesn’t rely on one central authority. Instead, nodes and validators follow shared rules to agree on which transactions and blocks are valid. This agreement is reached through a consensus algorithm. The two most widely used are Proof of Work and Proof of Stake.
Proof of Work (PoW)
In Proof of Work, miners compete to solve complex mathematical puzzles. The first miner to solve the puzzle adds the new block to the blockchain and earns a block reward. Other nodes verify the solution before accepting it. Miners often work together in mining pools to improve their chances of earning rewards. Proof of Work keeps the network very secure, but requires significant energy and computing power. Bitcoin continues to use this method.
Proof of Stake (PoS)
Proof of Stake typically selects validators based on how many coins they have locked up (staked) in the network. The more stake a validator has, the more likely they may be to propose or validate blocks, depending on the network. Validators take turns proposing and confirming new blocks. If a validator attempts to cheat, they risk losing their staked coins through a penalty mechanism known as slashing. PoS is now used by Ethereum (which switched from PoW in September 2022 via The Merge), BNB Chain, and Solana, among others.
Why Does Verification Matter?
Two problems historically made digital currencies difficult to build:
Double-spending: the same funds being sent to two or more recipients simultaneously.
Dependence on central authorities: requiring banks or companies to approve and record every transaction.
The blockchain solves both problems. Every transaction is recorded openly and permanently, so the same cryptocurrency cannot be spent twice. And instead of a single controlling institution, thousands of computers work together to verify each transaction, making the system highly resistant to fraud and manipulation.
What Is a Blockchain Confirmation?
A confirmation occurs each time a new block is added on top of the block containing your transaction. The more confirmations a transaction has, the harder it becomes to reverse.
Different blockchains require different numbers of confirmations before a transaction is considered settled. Bitcoin typically requires 3 to 6 confirmations for high-value transfers, as each new block makes a past transaction progressively harder to reverse. On Ethereum, some services wait for a certain number of blocks, while others wait for finality, which typically occurs after a few minutes under normal conditions.
FAQ
How is a Bitcoin transaction verified?
When you send Bitcoin, your transaction is broadcast to a network of nodes that check your digital signature and confirm you have sufficient funds. Valid transactions enter the mempool, where miners select them to include in a new block. A miner solves a computational puzzle to add the block to the chain and earn a block reward. Most platforms wait for 6 confirmations (roughly 1 hour) before treating a Bitcoin transaction as fully settled.
What is a mempool?
The mempool (memory pool) is a temporary holding area where valid but unconfirmed transactions wait to be picked up by a miner or validator and included in the next block. Transactions with higher fees are usually processed first. During periods of high network activity, mempool congestion can cause delays and increase the fees needed for timely confirmation.
What is the difference between Proof of Work and Proof of Stake?
Proof of Work requires miners to expend computing power to solve mathematical puzzles, with the winner adding the next block. Proof of Stake selects validators based on the amount of cryptocurrency they have staked, removing the need for energy-intensive computation. PoS is generally faster and more energy-efficient, while PoW is considered highly battle-tested for security. Bitcoin uses PoW; Ethereum switched to PoS in September 2022.
How many confirmations does a cryptocurrency transaction need?
The required number of confirmations varies by blockchain and transaction value. Bitcoin transactions are commonly considered settled after 6 confirmations (roughly 1 hour). Ethereum, using Proof of Stake, achieves finality in approximately 12 to 15 minutes through a checkpoint system rather than a confirmation count. Smaller or lower-value transfers may be accepted with fewer confirmations depending on the platform.
Can a confirmed cryptocurrency transaction be reversed?
Once a transaction has received sufficient confirmations, reversing it is extremely difficult. Each additional block added after your transaction makes it computationally harder (in PoW networks) or economically costlier (in PoS networks) to alter the historical record. While very deep reorganisations are theoretically possible, they are considered practically infeasible for established networks like Bitcoin and Ethereum under normal conditions.
Closing Thoughts
Transaction verification is what makes cryptocurrency trustworthy without a central authority. Whether through the computational effort of Proof of Work or the economic commitment of Proof of Stake, these mechanisms protect the network from fraud and double-spending. Understanding how verification works, including the role of the mempool, confirmation depth, and the trade-offs between consensus approaches, can help you better appreciate why different blockchains make different design decisions and how they affect transaction speed, cost, and security.
Further Reading
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