Let’s say a hacker wanted to change a transaction that happened 60 minutes, or six blocks, ago—maybe to remove evidence that she had spent some bitcoins, so she could spend them again. Her first step would be to go in and change the record for that transaction. Then, because she had modified the block, she would have to solve a new proof-of-work problem—find a new nonce—and do all of that computational work, all over again. (Again, due to the unpredictable nature of hash functions, making the slightest change to the original block means starting the proof of work from scratch.) From there, she’d have to start building an alternative chain going forward, solving a new proof-of-work problem for each block until she caught up with the present.
Mining a block is difficult because the SHA-256 hash of a block's header must be lower than or equal to the target in order for the block to be accepted by the network. This problem can be simplified for explanation purposes: The hash of a block must start with a certain number of zeros. The probability of calculating a hash that starts with many zeros is very low, therefore many attempts must be made. In order to generate a new hash each round, a nonce is incremented. See Proof of work for more information.
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To cut through some of the confusion surrounding bitcoin, we need to separate it into two components. On the one hand, you have bitcoin-the-token, a snippet of code that represents ownership of a digital concept – sort of like a virtual IOU. On the other hand, you have bitcoin-the-protocol, a distributed network that maintains a ledger of balances of bitcoin-the-token. Both are referred to as "bitcoin."

A bitcoin is defined by a sequence of digitally signed transactions that began with the bitcoin's creation, as a block reward. The owner of a bitcoin transfers it by digitally signing it over to the next owner using a bitcoin transaction, much like endorsing a traditional bank check. A payee can examine each previous transaction to verify the chain of ownership. Unlike traditional check endorsements, bitcoin transactions are irreversible, which eliminates risk of chargeback fraud.


The receiver of the first bitcoin transaction was cypherpunk Hal Finney, who created the first reusable proof-of-work system (RPOW) in 2004.[21] Finney downloaded the bitcoin software on its release date, and on 12 January 2009 received ten bitcoins from Nakamoto.[22][23] Other early cypherpunk supporters were creators of bitcoin predecessors: Wei Dai, creator of b-money, and Nick Szabo, creator of bit gold.[24] In 2010, the first known commercial transaction using bitcoin occurred when programmer Laszlo Hanyecz bought two Papa John's pizzas for 10,000 bitcoin.[25]
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