Nakamoto is estimated to have mined one million bitcoins[27] before disappearing in 2010, when he handed the network alert key and control of the code repository over to Gavin Andresen. Andresen later became lead developer at the Bitcoin Foundation.[28][29] Andresen then sought to decentralize control. This left opportunity for controversy to develop over the future development path of bitcoin.[30][29]

On 1 August 2017, a hard fork of bitcoin was created, known as Bitcoin Cash.[107] Bitcoin Cash has a larger block size limit and had an identical blockchain at the time of fork. On 24 October 2017 another hard fork, Bitcoin Gold, was created. Bitcoin Gold changes the proof-of-work algorithm used in mining, as the developers felt that mining had become too specialized.[108]
Armory is the most mature, secure and full featured Bitcoin wallet but it can be technologically intimidating for users. Whether you are an individual storing $1,000 or institution storing $1,000,000,000 this is the most secure option available. Users are in complete control all Bitcoin private keys and can setup a secure offline-signing process in Armory.
Instead of using an order book, OTC desks connect buy and sell orders directly between people. Desks are most commonly used for buying or selling incredibly large quantities of bitcoin, often surpassing millions of dollars in value. Some OTC desks even require a minimum trade value. Often, they are used specifically because a purchase or sale of such a large quantity need not affect the order books of exchanges. If a large order were to be filled on an exchange’s order book, it would significantly move the price of bitcoin. This is sometimes unfavorable for someone looking to buy or sell bitcoins without shifting the market price or without drawing attention to the transaction (OTC desks are not required to publicly disclose purchases).
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.
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For example, Ethereum (CCY: ETH-USD), which has a nearly $116 billion market cap and is the second-largest cryptocurrency behind bitcoin, currently has 200 organizations testing a version of its blockchain technology. Yes, traditional banks are testing out Ethereum's blockchain, but so are companies in the technology and energy industries. Integrated oil and gas giant BP (NYSE:BP) envisions using a version of Ethereum's blockchain to aid it with energy futures trading. If these transactions were to settle faster, BP could presumably improve its margin. 
Let's say you had one legit $20 and one really good photocopy of that same $20. If someone were to try to spend both the real bill and the fake one, someone who took the trouble of looking at both of the bills' serial numbers would see that they were the same number, and thus one of them had to be false. What a Bitcoin miner does is analogous to that--they check transactions to make sure that users have not illegitimately tried to spend the same Bitcoin twice. This isn't a perfect analogy--we'll explain in more detail below.
Although transactions are publicly recorded on the blockchain, user data is not — or, at least not in full. In order to conduct transactions on the Bitcoin network, participants must run a program called a “wallet.” Each wallet consists of two unique and distinct cryptographic keys: a public key and a private key. The public key is the location where transactions are deposited to and withdrawn from. This is also the key that appears on the blockchain ledger as the user’s digital signature.
Evolving beyond the complex world of cryptocurrencies, blockchain applications are now showing enormous potential for many key industries. Industry analyst, Gartner, predicts that blockchain's business value-add will grow to US$176 billion by 2025.1 Although in its nascent stages and not without challenges, the technology is poised to revolutionize how consumers and businesses interact with data. Blockchain has the potential to redefine how we manage supply chains, maintain transactions and exchange assets.
Blockchain is the underlying technology behind cryptocurrencies like Bitcoin. Unlike physical currency, digital cash and cryptocurrencies come with a very real problem called Double-Spending. Let me explain what that is. When I email you a picture of my cat, I’m sending you a copy and not my original picture. However, when I need to send you money online, as much as I would love to send you a copy of it, it’s a bad idea if I really do that! With Bitcoin, there was a risk that the holder could just send copies of the same bitcoin token in different transactions, leading to “Double-Spending”.
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There is a definite need for better identity management on the web. The ability to verify your identity is the lynchpin of financial transactions that happen online. However, remedies for the security risks that come with web commerce are imperfect at best. Distributed ledgers offer enhanced methods for proving who you are, along with the possibility to digitize personal documents. Having a secure identity will also be important for online interactions — for instance, in the sharing economy. A good reputation, after all, is the most important condition for conducting transactions online.

Wallets and similar software technically handle all bitcoins as equivalent, establishing the basic level of fungibility. Researchers have pointed out that the history of each bitcoin is registered and publicly available in the blockchain ledger, and that some users may refuse to accept bitcoins coming from controversial transactions, which would harm bitcoin's fungibility.[118]
Transactions are defined using a Forth-like scripting language.[3]:ch. 5 Transactions consist of one or more inputs and one or more outputs. When a user sends bitcoins, the user designates each address and the amount of bitcoin being sent to that address in an output. To prevent double spending, each input must refer to a previous unspent output in the blockchain.[69] The use of multiple inputs corresponds to the use of multiple coins in a cash transaction. Since transactions can have multiple outputs, users can send bitcoins to multiple recipients in one transaction. As in a cash transaction, the sum of inputs (coins used to pay) can exceed the intended sum of payments. In such a case, an additional output is used, returning the change back to the payer.[69] Any input satoshis not accounted for in the transaction outputs become the transaction fee.[69]
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