Bitcoin is a cryptocurrency and a payment system:3 invented:3,4 by an unidentified programmer, or group of programmers, under the name of Satoshi Nakamoto. Bitcoin was introduced on 31 October 2008 to a cryptography mailing list, and released as open-source software in 2009. There have been various claims and speculation concerning the identity of Nakamoto, none of which are confirmed. The system is peer-to-peer and transactions take place between users directly, without an intermediary.:4 These transactions are verified by network nodes and recorded in a public distributed ledger called the blockchain, which uses bitcoin as its unit of account. Since the system works without a central repository or single administrator, the U.S. Treasury categorises bitcoin as a decentralised virtual currency. Bitcoin is often called the first cryptocurrency, although prior systems existed and it is more correctly described as the first decentralised digital currency. Bitcoin is the largest of its kind in terms of total market value.

Bitcoins are created as a reward in a competition in which users offer their computing power to verify and record bitcoin transactions into the blockchain. This activity is referred to as mining and successful miners are rewarded with transaction fees and newly created bitcoins.:5-7 Besides being obtained by mining, bitcoins can be exchanged for additional currencies, products, and services. When sending bitcoins, users can pay an optional transaction fee to the miners. This might expedite the transaction being confirmed.

In February 2015, the number of merchants accepting bitcoin for products and services passed 100,000. Instead of 2–3% typically imposed by credit card processors, merchants accepting bitcoins often pay fees in the range from zero percent to less than 2%. Despite the fourfold increase in the number of merchants accepting bitcoin in 2014, the cryptocurrency didn't have much momentum in retail transactions. The European Banking Authority and additional sources:11 have warned that bitcoin users aren't protected by refund rights or chargebacks. The use of bitcoin by criminals has attracted the attention of financial regulators, legislative bodies, law enforcement, and media. Criminal activities are primarily focused on darknet markets and theft, though officials in countries such as the United States additionally recognise that bitcoin can provide legitimate financial services.

Journalist Nikolai Hampton characterised bitcoin as a form of digital money that uses several cryptographic techniques "to secure transactions between untrusted third parties, without the need for a central authority or bank."

Etymology and orthography

The word bitcoin occurred in the white paper that defined bitcoin published in 2008. It is a compound of the words bit and coin. The white paper frequently uses the shorter coin.

There is no uniform convention for bitcoin capitalization. Some sources use Bitcoin, capitalized, to refer to the technology and network and bitcoin, lowercase, to refer to the unit of account. The Wall Street Journal, The Chronicle of Higher Education, and the Oxford English Dictionary advocate use of lowercase bitcoin in all cases. This article follows the latter convention.

Design

Blockchain

The blockchain is a public ledger that records bitcoin transactions. A novel solution accomplishes this without any trusted central authority: maintenance of the blockchain is performed by a network of communicating nodes running bitcoin software. Transactions of the form payer X sends Y bitcoins to payee Z are broadcast to this network using readily available software applications. Network nodes can validate transactions, add them to their copy of the ledger, and then broadcast these ledger additions to additional nodes. The blockchain is a distributed database – to achieve independent verification of the chain of ownership of any and every bitcoin (amount), each network node stores its own copy of the blockchain. Approximately six times per hour, a new group of accepted transactions, a block, is created, added to the blockchain, and quickly published to all nodes. This allows bitcoin software to determine when a particular bitcoin amount has been spent, which is necessary in order to prevent double-spending in an environment without central oversight. Whereas a conventional ledger records the transfers of actual bills or promissory notes that exist apart from it, the blockchain is the only place that bitcoins can be said to exist in the form of unspent outputs of transactions.

Units

The unit of account of the bitcoin system is bitcoin. As of 2014, symbols used to represent bitcoin are BTC, XBT, and .:2 Small amounts of bitcoin used as alternative units are millibitcoin (mBTC), microbitcoin (µBTC), and satoshi. Named in homage to bitcoin's creator, a satoshi is the smallest amount within bitcoin representing 0.00000001 bitcoin, one hundred millionth of a bitcoin. A millibitcoin equals to 0.001 bitcoin, one thousandth of bitcoin. One microbitcoin equals to 0.000001 bitcoin, one millionth of a bitcoin. A microbitcoin is at times referred to as a bit.

A proposal was submitted to the Unicode Consortium in October 2015 to add a codepoint for the symbol. It is in the pipeline for position 20BF (₿) in the Currency Symbols block.

Ownership

Ownership of bitcoins implies that a user can spend bitcoins associated with a specific address. To do so, a payer must digitally sign the transaction using the corresponding private key. Without knowledge of the private key, the transaction can't be signed and bitcoins can't be spent. The network verifies the signature using the public key.:ch. 5

If the private key is lost, the bitcoin network won't recognise any additional evidence of ownership; the coins are then unusable, and thus effectively lost. For example, in 2013 one user claimed to have lost 7,500 bitcoins, worth $7.5 million at the time, when he accidentally discarded a hard drive containing his private key.

Transactions

A transaction must have one or more inputs. For the transaction to be valid, every input must be an unspent output of a previous transaction. Every input must be digitally signed. The use of multiple inputs corresponds to the use of multiple coins in a cash transaction. A transaction can additionally have multiple outputs, allowing one to make multiple payments in one go. A transaction output can be specified as an arbitrary multiple of satoshi. 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. Any input satoshis not accounted for in the transaction outputs become the transaction fee.

Mining

Relative mining difficulty, the scale is logarithmic.
A mining farm in Iceland

Mining is a record-keeping service. Miners keep the blockchain consistent, complete, and unalterable by repeatedly verifying and collecting newly broadcast transactions into a new group of transactions called a block. Each block contains a cryptographic hash of the previous block, using the SHA-256 hashing algorithm,:ch. 7 which links it to the previous block thus giving the blockchain its name.

In order to be accepted by the rest of the network, a new block must contain a so-called proof-of-work. The proof-of-work requires miners to find a number called a nonce, such that when the block content is hashed along with the nonce, the result is numerically smaller than the network's difficulty target.:ch. 8 This proof is easy for any node in the network to verify, but extremely time-consuming to generate, as for a secure cryptographic hash, miners must try a large number of different nonce values (usually the sequence of tested values is 0, 1, 2, 3, …:ch. 8) before meeting the difficulty target.

Every 2016 blocks (approximately 14 days), the difficulty target is adjusted based on the network's recent performance, with the aim of keeping the average time between new blocks at ten minutes. In this way the system automatically adapts to the total amount of mining power on the network.:ch. 8

Between 1 March 2014 and 1 March 2015, the average number of nonces miners had to try before creating a new block increased from 16.4 quintillion to 200.5 quintillion.

The proof-of-work system, alongside the chaining of blocks, makes modifications of the blockchain extremely hard, as an attacker must modify all subsequent blocks in order for the modifications of one block to be accepted. As new blocks are mined all the time, the difficulty of modifying a block increases as time passes and the number of subsequent blocks (also called confirmations of the given block) increases.

Practicalities

It has become common for miners to join mining pools, which combine the computational resources of their members in order to increase the frequency of generating new blocks. The reward for each block is then split proportionately among the members, creating a more predictable stream of income for each miner without necessarily changing their long-term average income, although a fee might be charged for the service.

The rewards of mining have led to ever-more-specialized technology being utilized. The most efficient mining hardware makes use of custom designed application-specific integrated circuits, which outperform general-purpose CPUs while using less power. As of 2015, a miner who isn't using purpose-built hardware is unlikely to earn enough to cover the cost of the electricity used in their efforts, even if they're a member of a pool.

Energy consumption

In 2013, Mark Gimein estimated electricity use to be about 982 megawatt-hours a day, which can be used to power roughly 31,000 US homes. In 2014, Karl J. O'Dwyer and David Malone estimated that specialised computers mining bitcoins required 0.1 to 10 GW of power. As of 2015, The Economist estimated that even if all miners used modern facilities, the combined electricity consumption would be 1.46 terawatt-hours per year—equal to the consumption of about 135,000 American homes.

Bitcoin miners have set up in places like Iceland where geothermal energy is cheap and cooling Arctic air is free. Journalist Matt O'Brien has pointed out that pollution connected with electricity usage in "energy-intensive" bitcoin mining are "paying less than they should" due to "environmental spillovers on everyone else, or what economists call negative externalities." Thus bitcoin isn't lowering costs, but "shifting them" into pollution costs.

Supply

Total bitcoins in circulation.

The successful miner finding the new block is rewarded with newly created bitcoins and transaction fees. As of 9 July 2016, the reward amounted to 12.5 newly created bitcoins per block added to the blockchain. To claim the reward, a special transaction called a coinbase is included with the processed payments.:ch. 8 All bitcoins in existence have been created in such coinbase transactions. The bitcoin protocol specifies that the reward for adding a block will be halved every 210,000 blocks (approximately every four years). Eventually, the reward will decrease to zero, and the limit of 21 million bitcoins will be reached c. 2140; the record keeping will then be rewarded by transaction fees solely.

In additional words, bitcoin's inventor Nakamoto set a monetary policy at the start of the bitcoin concept that there would only ever be 21 million bitcoins in total, their numbers being released roughly every ten minutes, and the rate at which they would be generated would drop by half every four years until all were in circulation.

Transaction fees

Paying a transaction fee is optional. Miners can choose which transactions to process and prioritise those that pay higher fees.

Wallets

Electrum bitcoin wallet
Bitcoin paper wallet generated at bitaddress.org
Trezor hardware wallet

A wallet stores the information necessary to transact bitcoins. While wallets are often described as a place to hold or store bitcoins, due to the nature of the system, bitcoins are inseparable from the blockchain transaction ledger. A better way to describe a wallet is something that "stores the digital credentials for your bitcoin holdings" and allows you to access (and spend) them. Bitcoin uses public-key cryptography, in which two cryptographic keys, one public and one private, are generated. At its most basic, a wallet is a collection of these keys.

There are several types of wallets. Software wallets connect to the network and allow spending bitcoins in addition to holding the credentials that prove ownership. Software wallets can be split further in two categories: full clients and lightweight clients.

  • Full clients verify transactions directly on a local copy of the blockchain (over 80 GB as of November 2016). Because of its size / complexity, the entire blockchain isn't suitable for all computing devices.
  • Lightweight clients on the additional hand consult a full client to send and receive transactions without requiring a local copy of the entire blockchain (see simplified payment verification – SPV). This makes lightweight clients much faster to setup and allows them to be used on low-power, low-bandwidth devices such as smartphones. When using a lightweight wallet however, the user must trust the server to a certain degree. When using a lightweight client, the server can not steal bitcoins, but it can report faulty values back to the user. With both types of software wallets, the users are responsible for keeping their private keys in a secure place.

Besides software wallets, Internet services called online wallets offer similar functionality but might be easier to use. In this case, credentials to access funds are stored with the online wallet provider rather than on the user's hardware. As a result, the user must have complete trust in the wallet provider. A malicious provider or a breach in server security might cause entrusted bitcoins to be stolen. An example of such security breach occurred with Mt. Gox in 2011.

Physical wallets additionally exist and are more secure, as they store the credentials necessary to spend bitcoins offline. Examples combine a novelty coin with these credentials printed on metal, Others are simply paper printouts. An Additional type of wallet called a hardware wallet keeps credentials offline while facilitating transactions.

Reference implementation

The first wallet programme was released in 2009 by Satoshi Nakamoto as open-source code. Sometimes referred to as the "Satoshi client," this is additionally known as the reference client because it serves to define the bitcoin protocol and acts as a standard for additional implementations. In version 0.5 the client moved from the wxWidgets user interface toolkit to Qt, and the whole bundle was referred to as Bitcoin-Qt. After the release of version 0.9, the software bundle was renamed Bitcoin Core to distinguish itself from the network. Today, additional forks of Bitcoin Core exist such as Bitcoin XT, Bitcoin Classic, and Bitcoin Unlimited.

Privacy

Bitcoin is a pseudonymous currency, meaning that funds aren't tied to real-world entities but rather bitcoin addresses. Owners of bitcoin addresses aren't explicitly identified, but all transactions on the blockchain are public. In addition, transactions can be linked to individuals and companies through "idioms of use" (e.g., transactions that spend coins from multiple inputs indicate that the inputs might have a common owner) and corroborating public transaction data with known information on owners of certain addresses. Additionally, bitcoin exchanges, where bitcoins are traded for traditional currencies, might be required by law to gather personal information.

To heighten financial privacy, a new bitcoin address can be generated for each transaction. For example, hierarchical deterministic wallets generate pseudorandom "rolling addresses" for every transaction from a single seed, while only requiring a single passphrase to be remembered to recover all corresponding private keys. Additionally, "mixing" and CoinJoin services aggregate multiple users' coins and output them to fresh addresses to increase privacy. Researchers at Stanford University and Concordia University have additionally shown that bitcoin exchanges and additional entities can prove assets, liabilities, and solvency without revealing their addresses using zero-knowledge proofs.

According to Dan Blystone, "Ultimately, bitcoin resembles cash as much as it does credit cards."

Fungibility

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 a few users might refuse to accept bitcoins coming from controversial transactions, which would harm bitcoin's fungibility. Projects such as Zerocoin and Dark Wallet aim to address these privacy and fungibility issues.

History

Number of bitcoin transactions per month (logarithmic scale).
Liquidity (estimated, USD/year, logarithmic scale).

Bitcoin was created by Satoshi Nakamoto, who published the invention on 31 October 2008 in a research paper called "Bitcoin: A Peer-to-Peer Electronic Cash System". It was implemented as open source code and released in January 2009. Bitcoin is often called the first cryptocurrency although prior systems existed. Bitcoin is more correctly described as the first decentralised digital currency.

One of the first supporters, adopters, contributor to bitcoin and receiver of the first bitcoin transaction was programmer Hal Finney. Finney downloaded the bitcoin software the day it was released, and received 10 bitcoins from Nakamoto in the world's first bitcoin transaction. Other early supporters were Wei Dai, creator of bitcoin predecessor b-money, and Nick Szabo, creator of bitcoin predecessor bit gold.

In the early days, Nakamoto is estimated to have mined 1 million bitcoins. Before disappearing from any involvement in bitcoin, Nakamoto in a sense handed over the reins to developer Gavin Andresen, who then became the bitcoin lead developer at the Bitcoin Foundation, the 'anarchic' bitcoin community's closest thing to an official public face.

Based on bitcoin's open source code, additional cryptocurrencies started to emerge in 2011.

In March 2013, a technical glitch caused a fork in the blockchain, with one half of the network adding blocks to one version of the chain and the additional half adding to another. For six hours two bitcoin networks operated at the same time, each with its own version of the transaction history. The core developers called for a temporary halt to transactions, sparking a sharp sell-off. Normal operation was restored when the majority of the network downgraded to version 0.7 of the bitcoin software.

In 2013 a few mainstream websites began accepting bitcoins. WordPress had started in November 2012, followed by OKCupid in April 2013, TigerDirect and Overstock.com in January 2014, Expedia in June 2014, Newegg and Dell in July 2014, and Microsoft in December 2014. The Electronic Frontier Foundation, a non-profit group, started accepting bitcoins in January 2011, stopped accepting them in June 2011, and began again in May 2013.

In May 2013, the Department of Homeland Security seized assets belonging to the Mt. Gox exchange. The U.S. Federal Bureau of Investigation (FBI) shut down the Silk Road website in October 2013.

In October 2013, Chinese internet giant Baidu had allowed clients of website security services to pay with bitcoins. During November 2013, the China-based bitcoin exchange BTC China overtook the Japan-based Mt. Gox and the Europe-based Bitstamp to become the largest bitcoin trading exchange by trade volume. On 19 November 2013, the value of a bitcoin on the Mt. Gox exchange soared to a peak of US$900 after a United States Senate committee hearing was told by the FBI that virtual currencies are a legitimate financial service. On the same day, one bitcoin traded for over RMB¥6780 (US$1,100) in China. On 5 December 2013, the People's Bank of China prohibited Chinese financial institutions from using bitcoins. After the announcement, the value of bitcoins dropped, and Baidu no longer accepted bitcoins for certain services. Buying real-world goods with any virtual currency has been illegal in China after at least 2009.

The first bitcoin ATM was installed in October 2013 in Vancouver, British Columbia, Canada.

With about 12 million existing bitcoins in November 2013, the new price increased the market cap for bitcoin to at least US$7.2 billion. By 23 November 2013, the total market capitalization of bitcoin exceeded US$10 billion for the first time.

In the U.S., two men were arrested in January 2014 on charges of money-laundering using bitcoins; one was Charlie Shrem, the head of now defunct bitcoin exchange BitInstant and a vice chairman of the Bitcoin Foundation. Shrem allegedly allowed the additional arrested party to purchase large quantities of bitcoins for use on black-market websites.

In early February 2014, one of the largest bitcoin exchanges, Mt. Gox, suspended withdrawals citing technical issues. By the end of the month, Mt. Gox had filed for bankruptcy protection in Japan amid reports that 744,000 bitcoins had been stolen. Months before the filing, the popularity of Mt. Gox had waned as users experienced difficulties withdrawing funds.

On 18 June 2014, it was announced that bitcoin payment service provider BitPay would become the new sponsor of St. Petersburg Bowl under a two-year deal, renamed the Bitcoin St. Petersburg Bowl. Bitcoin was to be accepted for ticket and concession sales at the game as part of the sponsorship, and the sponsorship itself was additionally paid for using bitcoin.

Less than one year after the collapse of Mt. Gox, United Kingdom-based exhange Bitstamp announced that their exchange would be taken offline while they investigate a hack which resulted in about 19,000 bitcoins (equivalent to roughly US$5 million at that time) being stolen from their hot wallet. The exchange remained offline for several days amid speculation that customers had lost their funds. Bitstamp resumed trading on 9 January after increasing security measures and assuring customers that their account balances wouldn't be impacted.

The bitcoin exchange service Coinbase launched the first regulated bitcoin exchange in 25 US states on 26 January 2015. At the time of the announcement, CEO Brian Armstrong stated that Coinbase intends to expand to thirty countries by the end of 2015. A spokesperson for Benjamin M. Lawsky, the superintendent of New York state's Department of Financial Services, stated that Coinbase is operating without a licence in the state of New York. Lawsky is responsible for the development of the so-called 'BitLicense', which companies need to acquire in order to legally operate in New York.

In August 2015 it was announced that Barclays would become the first UK high street bank to start accepting bitcoin, with the bank revealing that it plans to allow users to make charitable donations using the currency.

A major bitcoin exchange, Bitfinex, was hacked and nearly 120,000 BTC (around $60m) was stolen in 2016.

Economics

Classification

According to the director of the Institute for Money, Technology and Financial Inclusion at the University of California-Irvine there's "an unsettled debate about whether bitcoin is a currency". Bitcoin is commonly referred to with terms like: digital currency,:1 digital cash, virtual currency, electronic currency, or cryptocurrency. Its inventor, Satoshi Nakamoto, used the term electronic cash. Bitcoins have three useful qualities in a currency, according to The Economist in January 2015: they're "hard to earn, limited in supply and easy to verify". Economists define money as a store of value, a medium of exchange, and a unit of account and agree that bitcoin has a few way to go to meet all these criteria. It does best as a medium of exchange, as of February 2015 the number of merchants accepting bitcoin has passed 100,000. As of March 2014, the bitcoin market suffered from volatility, limiting the ability of bitcoin to act as a stable store of value, and retailers accepting bitcoin use additional currencies as their principal unit of account.

Journalists and academics additionally debate what to call bitcoin. Some media outlets do make a distinction between "real" money and bitcoins, while others call bitcoin real money. The Wall Street Journal declared it a commodity in December 2013. A Forbes journalist referred to it as digital collectible. Two University of Amsterdam computer scientists proposed the term "money-like informational commodity". The Wall Street Journal, Wired, Daily Mail Australia, Forbes, Business Wire and Reuters used the digital asset classification for bitcoin. In a 2016 Forbes article, bitcoin was characterised as a member of a new asset class.

The People's Bank of China has stated that bitcoin "is fundamentally not a currency but an investment target".

In addition to the above, bitcoin is additionally characterised as a payment system.:1

Decentralization

Per sources such as the academic Mercatus Center, U.S. Treasury, Reuters, The Washington Post, The Daily Herald, The New Yorker, and others, bitcoin is decentralized.

There is, however, additionally an article that appeared in IEEE Security & Privacy magazine, in which it is stated that "contrary to widespread belief, it isn’t truly decentralised as it's deployed and implemented today."

According to a self-published article by Dean Walsh, "Crucially, it is important to recognise that bitcoin isn't entirely decentralized, and that the degree to which it is can vary over time and be influenced by a wide range of different factors."

Buying and selling

A Bitcoin ATM in California.

Bitcoins can be bought and sold both on- and offline. Participants in online exchanges offer bitcoin buy and sell bids. Using an online exchange to obtain bitcoins entails a few risk, and, according to a study published in April 2013, 45 percent of exchanges fail and take client bitcoins with them. Exchanges have after implemented measures to provide proof of reserves in an effort to convey transparency to users. Offline, bitcoins might be purchased directly from an individual or at a bitcoin ATM. Bitcoin machines aren't however traditional ATMs. Bitcoin kiosks are machines connected to the Internet, allowing the insertion of cash in exchange for bitcoins. Bitcoin kiosks don't connect to a bank and might additionally charge transaction fees as high as seven percent and exchange rates $50 over rates from elsewhere.

Price and volatility

Price (left vertical axis, logarithmic scale) and volatility (right vertical axis).

According to Mark T. Williams, as of 2014, bitcoin has volatility seven times greater than gold, eight times greater than the S&P 500, and eighteen times greater than the U.S. dollar.

Attempting to explain the high volatility, a group of Japanese scholars stated that there's no stabilisation mechanism. The Bitcoin Foundation contends that high volatility is due to insufficient liquidity, while a Forbes journalist claims that it is related to the uncertainty of its long-term value, and the high volatility of a startup currency makes sense, "because people are still experimenting with the currency to figure out how useful it is."

There are uses where volatility doesn't matter, such as online gambling, tipping, and international remittances. As of 2014, pro-bitcoin venture capitalists argued that the greatly increased trading volume that planned high-frequency trading exchanges would generate is needed to decrease price volatility.

The price of bitcoins has gone through various cycles of appreciation and depreciation referred to by a few as bubbles and busts. In 2011, the value of one bitcoin rapidly rose from about US$0.30 to US$32 before returning to US$2. In the latter half of 2012 and throughout the 2012–13 Cypriot financial crisis, the bitcoin price began to rise, reaching a high of US$266 on 10 April 2013, before crashing to around US$50. On 29 November 2013, the cost of one bitcoin rose to the all-time peak of US$1,242. In 2014, the price fell sharply, and as of April remained depressed at little more than half 2013 prices. As of August 2014 it was under US$600. In January 2015, noting that the bitcoin price had dropped to its lowest level after spring 2013 – around US$224 – The New York Times suggested that "[w]ith no signs of a rally in the offing, the industry is bracing for the effects of a prolonged decline in prices. In particular, bitcoin mining companies, which are essential to the currency's underlying technology, are flashing warning signs." Also in January 2015, Business Insider reported that deep web drug dealers were "freaking out" as they lost profits through being unable to convert bitcoin revenue to cash quickly enough as the price declined – and that there was a danger that dealers selling reserves to stay in business might force the bitcoin price down further.

On 4 November 2015, bitcoin had risen by more than 20%, exceeding $490. The Financial Times associated the rapid growth with the popularity of "socio-financial networks" MMM operated by Russian businessman Sergei Mavrodi.

Speculative bubble dispute

Bitcoin has been labelled a speculative bubble by a large number of including former Fed Chairman Alan Greenspan and economist John Quiggin. Nobel Memorial Prize laureate Robert Shiller said that bitcoin "exhibited a large number of of the characteristics of a speculative bubble". Two lead software developers of bitcoin, Gavin Andresen and Mike Hearn, have warned that bubbles might occur. David Andolfatto, a vice president at the Federal Reserve Bank of St. Louis, stated, "Is bitcoin a bubble? Yes, if bubble is defined as a liquidity premium." According to Andolfatto, the price of bitcoin "consists purely of a bubble," but he concedes that a large number of assets have prices that are greater than their intrinsic value.:21 Journalist Matthew Boesler rejects the speculative bubble label and sees bitcoin's quick rise in price as nothing more than normal economic forces at work. The Washington Post pointed out that the observed cycles of appreciation and depreciation don't correspond to the definition of speculative bubble.

Ponzi scheme concerns

Various journalists, economists, and the central bank of Estonia have voiced concerns that bitcoin is a Ponzi scheme. Eric Posner, a law professor at the University of Chicago, stated in 2013 that "a real Ponzi scheme takes fraud; bitcoin, by contrast, seems more like a collective delusion." In 2014 reports by both the World Bank and the Swiss Federal Council examined the concerns and came to the conclusion that bitcoin isn't a Ponzi scheme.:7:21 Notwithstanding bitcoin is still 'a little bit' of a Ponzi sceme or pyramid scheme according to a few mainstream writers such as Matt O Brien of the Washington Post who said its early adopters are trying to cash in on the idea of getting more people involved, driving the bitcoin price up and increasing their funds.

Value forecasts

Financial journalists and analysts, economists, and investors have attempted to predict the possible future value of bitcoin. In April 2013, economist John Quiggin stated, "bitcoins will attain their true value of zero sooner or later, but it is impossible to say when". A similar forecast was made in November 2014 by economist Kevin Dowd.

In December 2013, finance professor Mark T. Williams forecast a bitcoin would be worth less than $10 by July 2014. In the indicated period bitcoin has exchanged as low as $344 (April 2014) and throughout July 2014 the bitcoin low was $609. In December 2014, Williams said, "The probability of success is low, but if it does hit, the reward will be quite large."

In November 2014, David Yermack, Professor of Finance at New York University Stern School of Business, forecast that in November 2015 bitcoin might be all but worthless. In the indicated period bitcoin has exchanged as low as $176.50 (January 2015) and throughout November 2015 the bitcoin low was $309.90.

In May 2013, Bank of America FX and Rate Strategist David Woo forecast a maximum fair value per bitcoin of $1,300.

Obituaries

The "death" of bitcoin has been proclaimed numerous times. One journalist has recorded 29 such "obituaries" as of early 2015.

Forbes magazine declared bitcoin "dead" in June 2011, followed by Gizmodo Australia in August 2011. Wired magazine wrote it had "expired" in December 2012. Ouishare Magazine declared, "game over, bitcoin" in May 2013, and New York Magazine stated bitcoin was "on its path to grave" in June 2013. Reuters published an "obituary" for bitcoin in January 2014. Street Insider declared bitcoin "dead" in February 2014, followed by The Weekly Standard in March 2014, Salon in March 2014, Vice News in March 2014, and Financial Times in September 2014. In January 2015, USA Today states bitcoin was "headed to the ash heap", and The Telegraph declared "the end of bitcoin experiment". In January 2016, former bitcoin developer Mike Hearn called bitcoin a "failed project".

Peter Greenhill, Director of E-Business Development for the Isle of Man, commenting on the obituaries paraphrased Mark Twain saying "reports of bitcoin's death have been greatly exaggerated".

Reception

Some economists have responded positively to bitcoin while others have expressed skepticism. François R. Velde, Senior Economist at the Chicago Fed described it as "an elegant solution to the problem of creating a digital currency". Paul Krugman and Brad DeLong have found fault with bitcoin questioning why it should act as a reasonably stable store of value or whether there's a floor on its value. Economist John Quiggin has criticised bitcoin as "the final refutation of the efficient-market hypothesis".

David Andolfatto, Vice President at the Federal Reserve Bank of St. Louis, stated that bitcoin is a threat to the establishment, which he argues is a good thing for the Federal Reserve System and additional central banks because it prompts these institutions to operate sound policies.:33

Free software movement activist Richard Stallman has criticised the lack of anonymity and called for reformed development. PayPal President David A. Marcus calls bitcoin a "great place to put assets" but claims it won't be a currency until price volatility is reduced. Bill Gates, in relation to the cost of moving money from place to place in an interview for Bloomberg L.P. stated: "Bitcoin is exciting because it shows how cheap it can be."

Officials in countries such as Brazil, the Isle of Man, Jersey, the United Kingdom, and the United States have recognised its ability to provide legitimate financial services. Recent bitcoin developments have been drawing the interest of more financially savvy politicians and legislators as a result of bitcoin's capability to eradicate fraud, simplify transactions, and provide transparency, when bitcoins are properly utilized.

Acceptance by merchants

Bitcoins are accepted in this café in Delft in the Netherlands as of 2013

In 2015, the number of merchants accepting bitcoin exceeded 100,000. Instead of 2–3% typically imposed by credit card processors, merchants accepting bitcoins often pay fees in the range from zero percent to less than 2%. As of December 2014 select firms that accept payments in bitcoin include:

A Bitcoin ATM in Vienna – Westbahnhof
Alexa rankSite
33PayPal
41Microsoft
328Dell
329Newegg
505Overstock.com
512Expedia
1,981TigerDirect
5,674Dish Network
7,038Zynga
30,565Time Inc.
176,903PrivateFly
253,983Virgin Galactic
348,010Dynamite Entertainment
1,145,668Clearly Canadian
n.a.Sacramento Kings

Acceptance by nonprofits

The Electronic Frontier Foundation, Greenpeace, The Mozilla Foundation, and The Wikimedia Foundation. Some U.S. political candidates, including New York City Democratic Congressional candidate Jeff Kurzon have said they would accept campaign donations in bitcoin. In late 2013 the University of Nicosia became the first university in the world to accept bitcoins.

Payment service providers

Merchants accepting bitcoin such as Dish Network corp., use the services of bitcoin payment service providers such as BitPay or Coinbase. When a customer pays in bitcoin, the payment service provider accepts the bitcoin on behalf of the merchant, directly converts it, and sends the obtained amount to merchant's bank account, charging a fee of less than 1 percent for the service.

Use in retail transactions

Bitcoin ATM in The D Las Vegas Casino. An early retail adopter of bitcoin

Due to the design of bitcoin, all retail figures are only estimates. According to Tim Swanson, head of business development at a Hong Kong-based cryptocurrency technology company, in 2014, daily retail purchases made with bitcoin were worth about $2.3 million. He estimates that, as of February 2015, fewer than 5,000 bitcoins per day (worth roughly $1.2 million at the time) were being used for retail transactions, and concluded that in 2014 "it appears there has been quite little if any increase in retail purchases using bitcoin."

Financial institutions

Bitcoin companies have had difficulty opening traditional bank accounts because lenders have been leery of bitcoin's links to illicit activity. According to Antonio Gallippi, a co-founder of BitPay, "banks are scared to deal with bitcoin companies, even if they really want to". In 2014, the National Australia Bank closed accounts of businesses with ties to bitcoin, and HSBC refused to serve a hedge fund with links to bitcoin. Australian banks in general have been reported as closing down bank accounts of operators of businesses involving the currency; this has become the subject of an investigation by the Australian Competition and Consumer Commission. Nonetheless, Australian banks have keenly adopted the blockchain technology on which bitcoin is based.

In a 2013 report, Bank of America Merrill Lynch stated that "we believe bitcoin can become a major means of payment for e-commerce and might emerge as a serious competitor to traditional money-transfer providers."

In June 2014, the first bank that converts deposits in currencies instantly to bitcoin without any fees was opened in Boston.

As an investment

Some Argentinians have bought bitcoins to protect their savings against high inflation or the possibility that governments could confiscate savings accounts. During the 2012–2013 Cypriot financial crisis, bitcoin purchases in Cyprus rose due to fears that savings accounts would be confiscated or taxed. Other methods of investment are bitcoin funds. The first regulated bitcoin fund was established in Jersey in July 2014 and approved by the Jersey Financial Services Commission. Also, c. 2012 an attempt was made by the Winklevoss twins (who in April 2013 claimed they owned nearly one percent of all bitcoins in existence) to establish a bitcoin ETF. As of early 2015, they have announced plans to launch a New York-based bitcoin exchange named Gemini, which has received approval to launch on 5 October 2015. On 4 May 2015, Bitcoin Investment Trust started trading on the OTCQX market as GBTC. Forbes started publishing arguments in favour of investing in December 2015.

In 2013 and 2014, the European Banking Authority and the Financial Industry Regulatory Authority (FINRA), a United States self-regulatory organization, warned that investing in bitcoins carries significant risks. Forbes named bitcoin the best investment of 2013. In 2014, Bloomberg named bitcoin one of its worst investments of the year. In 2015, bitcoin topped Bloomberg's currency tables.

To improve access to price information and increase transparency, on 30 April 2014 Bloomberg LP announced plans to list prices from bitcoin companies Kraken and Coinbase on its 320,000 subscription financial data terminals. In May 2015, Intercontinental Exchange Inc., parent company of the New York Stock Exchange, announced a bitcoin index initially based on data from Coinbase transactions.

Venture capital

Venture capitalists, such as Peter Thiel's Founders Fund, which invested US$3 million in BitPay, don't purchase bitcoins themselves, instead funding bitcoin infrastructure like companies that provide payment systems to merchants, exchanges, wallet services, etc. In 2012, an incubator for bitcoin-focused start-ups was founded by Adam Draper, with financing help from his father, venture capitalist Tim Draper, one of the largest bitcoin holders after winning an auction of 30,000 bitcoins, at the time called 'mystery buyer'. The company's goal is to fund 100 bitcoin businesses within 2–3 years with $10,000 to $20,000 for a six percent stake. Investors additionally invest in bitcoin mining. According to a 2015 study by Paolo Tasca, bitcoin startups raised almost $1 billion in three years (Q1 2012 – Q1 2015).

Political economy

The decentralisation of money offered by virtual currencies like bitcoin has its theoretical roots in the Austrian school of economics, especially with Friedrich von Hayek in his book Denationalisation of Money: The Argument Refined, in which he advocates a complete free market in the production, distribution and management of money to end the monopoly of central banks.

Bitcoin appeals to tech-savvy libertarians, because it so far exists outside the institutional banking system and the control of governments. Notwithstanding researchers looking to uncover the reasons for interest in bitcoin didn't find evidence in Google search data that this was linked to libertarianism.

Bitcoin's appeal reaches from left wing critics, "who perceive the state and banking sector as representing the same elite interests, […] recognising in it the potential for collective direct democratic governance of currency" and socialists proposing their "own states, complete with currencies", to right wing critics suspicious of big government, at a time when activities within the regulated banking system were responsible for the severity of the financial crisis of 2007–08, "because governments aren't fully living up to the responsibility that comes with state-sponsored money". Bitcoin has been described as "remov[ing] the imbalance between the big boys of finance and the disenfranchised little man, potentially allowing early adopters to negotiate favourable rates on exchanges and transfers – something that only the quite biggest firms have traditionally enjoyed". Two WSJ journalists describe bitcoin in their book as "about freeing people from the tyranny of centralised trust".

Legal status and regulation

The legal status of bitcoin varies substantially from country to country and is still undefined or changing in a large number of of them. While a few countries have explicitly allowed its use and trade, others have banned or restricted it. Likewise, various government agencies, departments, and courts have classified bitcoins differently. Regulations and bans that apply to bitcoin probably extend to similar cryptocurrency systems.

In April 2013, Steven Strauss, a Harvard public policy professor, suggested that governments could outlaw bitcoin, and this possibility was mentioned again by a bitcoin investment vehicle in a July 2013 report to a regulator. Notwithstanding the vast majority of nations haven't done so as of 2014. It is illegal in Bangladesh, Bolivia, Ecuador.

In China in December 2013 the Chinese government declared that "bitcoin isn't a currency and shouldn't be circulated and used in the market as a currency." 'While people there are free to buy and sell it, financial institutions have been warned away'.

Criminal activity

The use of bitcoin by criminals has attracted the attention of financial regulators, legislative bodies, law enforcement, and the media. The FBI prepared an intelligence assessment, the SEC has issued a pointed warning about investment schemes using virtual currencies, and the U.S. Senate held a hearing on virtual currencies in November 2013.

Several news outlets have asserted that the popularity of bitcoins hinges on the ability to use them to purchase illegal goods. In 2014, researchers at the University of Kentucky found "robust evidence that computer programming enthusiasts and illegal activity drive interest in bitcoin, and find limited or no support for political and investment motives."

Funding Terrorism

Bitcoin is transparent in its reporting of how much gets transferred and when, but lacks oversight about reporting suspicious activities in comparison to the regulated banking system. Bitcoin provides 'cover' to 'move money around without triggering the usual alarm bells associated with giant transfers of cash.' Jennifer Shasky Calvery, former head of the US Treasury Department's Financial Crimes Enforcement Network (FinCEN), an agency that fights money laundering and terrorist finance, has stated that 'powerful payment technologies could facilitate bad actors.' She stated that "What keeps me up at night when I'm thinking about digital currency…the real threats out there, these days we’re thinking a lot about ISIL," Calvery said. "How they’re moving their money, and how potential US-based individuals are fitting foreign fighters: Are they moving their money, can we identify them from the movement of their money? What does it mean if they start moving their money through bitcoin? We’ve started to see a few public articles suggesting that has occurred."

Theft

There have been a large number of cases of bitcoin theft. One way this is accomplished involves a third party accessing the private key to a victim's bitcoin address, or of an online wallet. If the private key is stolen, all the bitcoins from the compromised address can be transferred. In that case, the network doesn't have any provisions to identify the thief, block further transactions of those stolen bitcoins, or return them to the legitimate owner.

Theft additionally occurs at sites where bitcoins are used to purchase illicit goods. In late November 2013, an estimated $100 million in bitcoins were allegedly stolen from the online illicit goods marketplace Sheep Marketplace, which immediately closed. Users tracked the coins as they were processed and converted to cash, but no funds were recovered and no culprits identified. A different black market, Silk Road 2, stated that throughout a February 2014 hack, bitcoins valued at $2.7 million were taken from escrow accounts.

Sites where users exchange bitcoins for cash or store them in "wallets" are additionally targets for theft. Inputs.io, an Australian wallet service, was hacked twice in October 2013 and lost more than $1 million in bitcoins. In late February 2014 Mt. Gox, one of the largest virtual currency exchanges, filed for bankruptcy in Tokyo amid reports that bitcoins worth $350 million had been stolen. Flexcoin, a bitcoin storage specialist based in Alberta, Canada, shut down on March 2014 after saying it discovered a theft of about $650,000 in bitcoins. Poloniex, a digital currency exchange, reported on March 2014 that it lost bitcoins valued at around $50,000. In January 2015 UK-based bitstamp, the third busiest bitcoin exchange globally, was hacked and $5 million in bitcoins were stolen. February 2015 saw a Chinese exchange named BTER lose bitcoins worth nearly $2 million to hackers.

A major bitcoin exchange, Bitfinex, was hacked and nearly 120,000 bitcoins (around $60m) was stolen in 2016. Bitfinex was forced to suspend its trading. The theft is the second largest bitcoin heist ever, dwarfed only by Mt. Gox theft in 2014. According to Forbes, "All of Bitfinex's customers,... will stand to lose money. The company has announced a haircut of 36.067% across the board."

Black markets

A CMU researcher estimated that in 2012, 4.5% to nine percent of all transactions on all exchanges in the world were for drug trades on a single dark web drugs market, Silk Road. Child pornography, murder-for-hire services, and weapons are additionally allegedly available on black market sites that sell in bitcoin. Due to the anonymous nature and the lack of central control on these markets, it is hard to know whether the services are real or just trying to take the bitcoins.

Several deep web black markets have been shut by authorities. In October 2013 Silk Road was shut down by U.S. law enforcement leading to a short-term decrease in the value of bitcoin. In 2015, the founder of the site was sentenced to life in prison. Alternative sites were soon available, and in early 2014 the Australian Broadcasting Corporation reported that the closure of Silk Road had little impact on the number of Australians selling drugs online, which had actually increased. In early 2014, Dutch authorities closed Utopia, an online illegal goods market, and seized 900 bitcoins. In late 2014, a joint police operation saw European and American authorities seize bitcoins and close 400 deep web sites including the illicit goods market Silk Road 2.0. Law enforcement activity has resulted in several convictions. In December 2014, Charlie Shrem was sentenced to two years in prison for indirectly helping to send $1 million to the Silk Road drugs site, and in February 2015, its founder, Ross Ulbricht, was convicted on drugs charges and faces a life sentence.

Some black market sites might seek to steal bitcoins from customers. The bitcoin community branded one site, Sheep Marketplace, as a scam when it prevented withdrawals and shut down after an alleged bitcoins theft. In a separate case, escrow accounts with bitcoins belonging to patrons of a different black market were hacked in early 2014.

According to the Internet Watch Foundation, a UK-based charity, bitcoin is used to purchase child pornography, and almost 200 such websites accept it as payment. Bitcoin isn't the sole way to purchase child pornography online, as Troels Oertling, head of the cybercrime unit at Europol, states, "Ukash and Paysafecard... have [also] been used to pay for such material." Notwithstanding the Internet Watch Foundation lists around 30 sites that exclusively accept bitcoins. Some of these sites have shut down, such as a deep web crowdfunding website that aimed to fund the creation of new child porn. Furthermore, hyperlinks to child porn websites have been added to the blockchain as arbitrary data can be included when a transaction is made.

Money laundering

Bitcoins might not be ideal for money laundering because all transactions are public. Authorities, including the European Banking Authority the FBI, and the Financial Action Task Force of the G7 have expressed concerns that bitcoin might be used for money laundering. In early 2014, an operator of a U.S. bitcoin exchange was arrested for money laundering. A report by UK's Treasury and Home Office named "UK national risk assessment of money laundering and terrorist financing" (2015 October) found that, of the twelve methods examined in the report, bitcoin carries the lowest risk of being used for money laundering, with the most common money laundering method being the banks.

Ponzi scheme

In a Ponzi scheme that utilised bitcoins, The Bitcoin Savings and Trust promised investors up to 7 percent weekly interest, and raised at least 700,000 bitcoins from 2011 to 2012. In July 2013 the U.S. Securities and Exchange Commission charged the company and its founder in 2013 "with defrauding investors in a Ponzi scheme involving bitcoin". In September 2014 the judge fined Bitcoin Savings & Trust and its owner $40 million for operating a bitcoin Ponzi scheme.

Malware

Malware stealing

Some malware can steal private keys for bitcoin wallets allowing the bitcoins themselves to be stolen. The most common type searches computers for cryptocurrency wallets to upload to a remote server where they can be cracked and their coins stolen. Many of these additionally log keystrokes to record passwords, often avoiding the need to crack the keys. A different approach detects when a bitcoin address is copied to a clipboard and quickly replaces it with a different address, tricking people into sending bitcoins to the wrong address. This method is effective because bitcoin transactions are irreversible.:57

One virus, spread through the Pony botnet, was reported in February 2014 to have stolen up to $220,000 in cryptocurrencies including bitcoins from 85 wallets. Security company Trustwave, which tracked the malware, reports that its latest version was able to steal 30 types of digital currency.

A type of Mac malware active in August 2013, Bitvanity posed as a vanity wallet address generator and stole addresses and private keys from additional bitcoin client software. A different trojan for Mac OS X, called CoinThief was reported in February 2014 to be responsible for multiple bitcoin thefts. The software was hidden in versions of a few cryptocurrency apps on Download.com and MacUpdate.

Unauthorized mining

In June 2011, Symantec warned about the possibility that botnets could mine covertly for bitcoins. Malware used the parallel processing capabilities of GPUs built into a large number of modern video cards. Although the average PC with an integrated graphics processor is virtually useless for bitcoin mining, tens of thousands of PCs laden with mining malware could produce a few results.

In mid-August 2011, bitcoin mining botnets were detected, and less than three months later, bitcoin mining trojans had infected Mac OS X.

In April 2013, electronic sports organisation E-Sports Entertainment was accused of hijacking 14,000 computers to mine bitcoins; the company later settled the case with the State of New Jersey.

German police arrested two people in December 2013 who customised existing botnet software to perform bitcoin mining, which police said had been used to mine at least $950,000 worth of bitcoins.

For four days in December 2013 and January 2014, Yahoo! Europe hosted an ad containing bitcoin mining malware that infected an estimated two million computers. The software, called Sefnit, was first detected in mid-2013 and has been bundled with a large number of software packages. Microsoft has been removing the malware through its Microsoft Security Essentials and additional security software.

Several reports of employees or students using university or research computers to mine bitcoins have been published.

Ransomware

Another type of bitcoin-related malware is ransomware. One programme called CryptoLocker, typically spread through legitimate-looking email attachments, encrypts the hard drive of an infected computer, then displays a countdown timer and demands a ransom, usually two bitcoins, to decrypt it. Massachusetts police said they paid a 2 bitcoin ransom in November 2013, worth more than $1,300 at the time, to decrypt one of their hard drives. Linkup, a combination ransomware and bitcoin mining programme that surfaced in February 2014, disables internet access and demands credit card information to restore it, while secretly mining bitcoins.

Security

Various potential attacks on the bitcoin network and its use as a payment system, real or theoretical, have been considered. The bitcoin protocol includes several features that protect it against a few of those attacks, such as unauthorised spending, double spending, forging bitcoins, and tampering with the blockchain. Other attacks, such as theft of private keys, require due care by users.

Unauthorized spending

Unauthorized spending is mitigated by bitcoin's implementation of public-private key cryptography. For example; when Alice sends a bitcoin to Bob, Bob becomes the new owner of the bitcoin. Eve observing the transaction might want to spend the bitcoin Bob just received, but she can't sign the transaction without the knowledge of Bob's private key.

Double spending

A specific problem that an internet payment system must solve is double-spending, whereby a user pays the same coin to two or more different recipients. An example of such a problem would be if Eve sent a bitcoin to Alice and later sent the same bitcoin to Bob. The bitcoin network guards against double-spending by recording all bitcoin transfers in a ledger (the blockchain) that's visible to all users, and ensuring for all transferred bitcoins that they haven't been previously spent.:4

Race attack

If Eve offers to pay Alice a bitcoin in exchange for goods and signs a corresponding transaction, it is still possible that she additionally creates a different transaction at the same time sending the same bitcoin to Bob. By the rules, the network accepts only one of the transactions. This is called a race attack, after there's a race which transaction will be accepted first. Alice can reduce the risk of race attack stipulating that she'll not deliver the goods until Eve's payment to Alice appears in the blockchain.

A variant race attack (which has been called a Finney attack by reference to Hal Finney) requires the participation of a miner. Instead of sending both payment requests (to pay Bob and Alice with the same coins) to the network, Eve issues only Alice's payment request to the network, while the accomplice tries to mine a block that includes the payment to Bob instead of Alice. There is a positive probability that the rogue miner will succeed before the network, in which case the payment to Alice will be rejected. As with the plain race attack, Alice can reduce the risk of a Finney attack by waiting for the payment to be included in the blockchain.

History modification

Each block that's added to the blockchain, starting with the block containing a given transaction, is called a confirmation of that transaction. Ideally, merchants and services that receive payment in bitcoin should wait for at least one confirmation to be distributed over the network, before assuming that the payment was done. The more confirmations that the merchant waits for, the more difficult it is for an attacker to successfully reverse the transaction in a blockchain—unless the attacker controls more than half the total network power, in which case it is called a 51 percent attack.

Deanonymisation of clients

Deanonymisation is a strategy in data mining in which anonymous data is cross-referenced with additional sources of data to re-identify the anonymous data source. Along with transaction graph analysis, which might reveal connexions between bitcoin addresses (pseudonyms), there's a possible attack which links a user's pseudonym to its IP address. If the peer is using Tor, the attack includes a method to separate the peer from the Tor network, forcing them to use their real IP address for any further transactions. The attack makes use of bitcoin mechanisms of relaying peer addresses and anti-DoS protection. The cost of the attack on the full bitcoin network is under €1500 per month.

Data in the blockchain

While it is possible to store any digital file in the blockchain, the larger the transaction size, the larger any associated fees become. Various items have been embedded, including URLs to child pornography, an ASCII art image of Ben Bernanke, material from the Wikileaks cables, prayers from bitcoin miners, and the original bitcoin whitepaper.

In academia

In the fall of 2014, undergraduate students at the Massachusetts Institute of Technology (MIT) each received bitcoins worth $100 "to better understand this emerging technology". The bitcoins weren't provided by MIT but rather the MIT Bitcoin Club, a student-run club.

In art, entertainment, and media

Films

A documentary film called The Rise and Rise of Bitcoin (late 2014) features interviews with people who use bitcoin, such as a computer programmer and a drug dealer.

Music

Several lighthearted songs celebrating bitcoin have been released.

Literature

In Charles Stross' science fiction novel Neptune's Brood (2014), a modification of bitcoin is used as the universal interstellar payment system. The functioning of the system is a major plot element of the book.

Television

In early 2015, the CNN series Inside Man featured an episode about bitcoin. Filmed in July 2014, it featured Morgan Spurlock living off of bitcoins for a week to figure out whether the world is ready for a new kind of money.