An early example, OpenBazaar uses the blockchain to create a peer-to-peer eBay. Download the app onto your computing device, and you can transact with OpenBazzar vendors without paying transaction fees. The “no rules” ethos of the protocol means that personal reputation will be even more important to business interactions than it currently is on eBay.
If you have been following banking, investing, or cryptocurrency over the last ten years, you may be familiar with “blockchain,” the record-keeping technology behind bitcoin. And there’s a good chance that it only makes so much sense. In trying to learn more about blockchain, you've probably encountered a definition like this: “blockchain is a distributed, decentralized, public ledger." The good news is, blockchain is actually easier to understand than that definition sounds.
* In a supply chain auditing blockchain application (https://blockgeeks.com/guides/what-is-blockchain-technology/), it’s said “a Provenance pilot project ensures that fish sold in Sushi restaurants in Japan has been sustainably harvested by its suppliers in Indonesia”. I am wondering how this can be done. How can blockchain validate the origin of the fish? Or an ethical diamond? There is no reliable IDs on the fish or the diamonds.
Once a transaction is recorded, its authenticity must be verified by the blockchain network. Thousands or even millions of computers on the blockchain rush to confirm that the details of the purchase are correct. After a computer has validated the transaction, it is added to the blockchain in the form of a block. Each block on the blockchain contains its own unique hash, along with the unique hash of the block before it. When the information on a block is edited in any way, that block’s hash code changes — however, the hash code on the block after it would not. This discrepancy makes it extremely difficult for information on the blockchain to be changed without notice.
When mining began, regular off-the-shelf PCs were fast enough to generate bitcoins. That's the way the system was set up—easier to mine in the beginning, harder to mine as more bitcoins are generated. Over the last few years, miners have had to move on to faster hardware in order to keep generating new bitcoins. Today, application-specific integrated circuits (ASIC) are being used. Programmer language aside, all this means is that the hardware is designed for one specific task—in this case mining.

Theoretically, it is possible for a hacker to take advantage of the majority rule in what is referred to as a 51% attack. Here’s how it would happen. Let’s say that there are 5 million computers on the Bitcoin network, a gross understatement for sure but an easy enough number to divide. In order to achieve a majority on the network, a hacker would need to control at least 2.5 million and one of those computers. In doing so, an attacker or group of attackers could interfere with the process of recording new transactions. They could send a transaction — and then reverse it, making it appear as though they still had the coin they just spent. This vulnerability, known as double-spending, is the digital equivalent of a perfect counterfeit and would enable users to spend their Bitcoins twice.


The bitcoin blockchain is a public ledger that records bitcoin transactions.[67] It is implemented as a chain of blocks, each block containing a hash of the previous block up to the genesis block[a] of the chain. A network of communicating nodes running bitcoin software maintains the blockchain.[31]:215–219 Transactions of the form payer X sends Y bitcoins to payee Z are broadcast to this network using readily available software applications.


Whether you’re an individual buying a lemonade or a multinational lemonade company selling your beverages, each transaction you add to the blockchain is checked against everyone else’s blockchain ledgers. This system prevents anyone from using the same bitcoin more than once—which was the biggest problem with all-digital currencies before bitcoin came along.
Bitcoin prices were negatively affected by several hacks or thefts from cryptocurrency exchanges, including thefts from Coincheck in January 2018, Coinrail and Bithumb in June, and Bancor in July. For the first six months of 2018, $761 million worth of cryptocurrencies was reported stolen from exchanges.[62] Bitcoin's price was affected even though other cryptocurrencies were stolen at Coinrail and Bancor, as investors worried about the security of cryptocurrency exchanges.[63][64][65]
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