Bitcoin Difficulty

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Financial institutions
Bitcoins can be bought on digital currency exchanges.

Per researchers, "there is little sign of bitcoin use" in international remittances despite high fees charged by banks and Western Union who compete in this market. The South China Morning Post, however, mentions the use of bitcoin by Hong Kong workers to transfer money home.

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.

On 10 December 2017, the Chicago Board Options Exchange started trading bitcoin futures, followed by the Chicago Mercantile Exchange, which started trading bitcoin futures on 17 December 2017.

In September 2019 the Central Bank of Venezuela, at the request of PDVSA, ran tests to determine if bitcoin and ether could be held in central bank's reserves. The request was motivated by oil company's goal to pay its suppliers.

As an investment
The Winklevoss twins have purchased bitcoin. In 2013, The Washington Post reported a claim that they owned 1% of all the bitcoins in existence at the time.

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.

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.

According to bitinfocharts.com, in 2017 there are 9,272 bitcoin wallets with more than $1 million worth of bitcoins. The exact number of bitcoin millionaires is uncertain as a single person can have more than one bitcoin wallet.

In August 2020, MicroStrategy invested in Bitcoin.

Venture capital
Peter Thiel's Founders Fund invested US$3 million in BitPay. In 2012, an incubator for bitcoin-focused start-ups was founded by Adam D*****r, with financing help from his father, venture capitalist Tim D*****r, 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 6% stake. Investors also 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).

Price and volatility
The price of bitcoins has gone through cycles of appreciation and depreciation referred to by some 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 during 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 a 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. As of 16 November 2020, the closing price of bitcoin equals US$16,717.

According to Mark T. Williams, as of 30 September 2014, bitcoin has volatility seven times greater than gold, eight times greater than the S%story%P 500, and 18 times greater than the US dollar. Hodl is a meme created in reference to holding (as opposed to selling) during periods of volatility.

Legal status, tax and regulation
Because of bitcoin's decentralized nature and its trading on online exchanges located in many countries, regulation of bitcoin has been difficult. However, the use of bitcoin can be criminalized, and shutting down exchanges and the peer-to-peer economy in a given country would constitute a de facto ban. The legal status of bitcoin varies substantially from country to country and is still undefined or changing in many of them. Regulations and bans that apply to bitcoin probably extend to similar cryptocurrency systems.

According to the Library of Congress, an "absolute ban" on trading or using cryptocurrencies applies in nine countries: Algeria, Bolivia, Egypt, Iraq, Morocco, Nepal, Pakistan, Vietnam, and the United Arab Emirates. An "implicit ban" applies in another 15 countries, which include Bahrain, Bangladesh, China, Colombia, the Dominican Republic, Indonesia, Kuwait, Lesotho, Lithuania, Macau, Oman, Qatar, Saudi Arabia and Taiwan.

In October 2020, the Islamic Republic News Agency announced pending regulations that would require bitcoin miners in Iran to sell bitcoin to the Central Bank of Iran, and the central bank would use it for imports. Iran, as of October 2020, had issued over 1,000 bitcoin mining licenses. The Iranian government initially took a stance against cryptocurrency, but later changed it after seeing that digital currency could be used to circumvent sanctions. The US Office of Foreign Assets Control listed two Iranians and their bitcoin addresses as part of its Specially Designated Nationals and Blocked Persons List for their role in the 2018 Atlanta cyberattack whose ransom was paid in bitcoin.

Regulatory warnings
The U.S. Commodity Futures Trading Commission has issued four "Customer Advisories" for bitcoin and related investments. A July 2018 warning emphasized that trading in any cryptocurrency is often speculative, and there is a risk of theft from hacking, and fraud. In May 2014 the U.S. Securities and Exchange Commission warned that investments involving bitcoin might have high rates of fraud, and that investors might be solicited on social media sites. An earlier "Investor Alert" warned about the use of bitcoin in Ponzi schemes.

The European Banking Authority issued a warning in 2013 focusing on the lack of regulation of bitcoin, the chance that exchanges would be hacked, the volatility of bitcoin's price, and general fraud. FINRA and the North American Securities Administrators Association have both issued investor alerts about bitcoin.

Price manipulation investigation
An official investigation into bitcoin traders was reported in May 2018. The U.S. Justice Department launched an investigation into possible price manipulation, including the techniques of spoofing and wash trades.

The U.S. federal investigation was prompted by concerns of possible manipulation during futures settlement dates. The final settlement price of CME bitcoin futures is determined by prices on four exchanges, Bitstamp, Coinbase, itBit and Kraken. Following the first delivery date in January 2018, the CME requested extensive detailed trading information but several of the exchanges refused to provide it and later provided only limited data. The Commodity Futures Trading Commission then subpoenaed the data from the exchanges.

State and provincial securities regulators, coordinated through the North American Securities Administrators Association, are investigating "bitcoin scams" and ICOs in 40 jurisdictions.

Academic research published in the Journal of Monetary Economics concluded that price manipulation occurred during the Mt Gox bitcoin theft and that the market remains vulnerable to manipulation. The history of hacks, fraud and theft involving bitcoin dates back to at least 2011.

Research by John M. Griffin and Amin Shams in 2018 suggests that trading associated with increases in the amount of the Tether cryptocurrency and associated trading at the Bitfinex exchange account for about half of the price increase in bitcoin in late 2017.

J.L. van der Velde, CEO of both Bitfinex and Tether, denied the claims of price manipulation: "Bitfinex nor Tether is, or has ever, engaged in any sort of market or price manipulation. Tether issuances cannot be used to prop up the price of bitcoin or any other coin/token on Bitfinex."

Criticism
The Bank for International Settlements summarized several criticisms of bitcoin in Chapter V of their 2018 annual report. The criticisms include the lack of stability in bitcoin's price, the high energy consumption, high and variable transactions costs, the poor security and fraud at cryptocurrency exchanges, vulnerability to debasement (from forking), and the influence of miners.

In 2015, The Economist described these criticisms as unfair, since bitcoin had been relatively stable during that year, and the shady image may have compelled users to overlook the capabilities of the blockchain technology.

As a speculative bubble
Bitcoin, along with other cryptocurrencies, has been described as an economic bubble by at least eight Nobel Memorial Prize in Economic Sciences laureates at various times, including Robert Shiller on 1 March 2014, Joseph Stiglitz on 29 November 2017, and Richard Thaler on 21 December 2017. On 29 January 2018, a noted Keynesian economist Paul Krugman has described bitcoin as "a bubble wrapped in techno-mysticism inside a cocoon of libertarian ideology", on 2 February 2018, professor Nouriel Roubini of New York University has called bitcoin the "mother of all bubbles", and on 27 April 2018, a University of Chicago economist James Heckman has compared it to the 17th-century tulip mania.

On 4 December 2013, Alan Greenspan referred to it as a "bubble" as did George Soros on 25 January 2018. Warren Buffett called bitcoin a "mirage" on 13 March 2014.

Energy consumption
See also: Consensus (computer science) § Some consensus protocols
Bitcoin has been criticized for the amount of electricity consumed by mining. As of 2015, The Economist estimated that even if all miners used modern facilities, the combined electricity consumption would be 166.7 megawatts (1.46 terawatt-hours per year). At the end of 2017, the global bitcoin mining activity was estimated to consume between one and four gigawatts of electricity. By 2018, bitcoin was estimated by Joule to use 2.55 GW, while Environmental Science %story% Technology estimated bitcoin to consume 3.572 GW (31.29 TWh for the year). In July 2019 BBC reported bitcoin consumes about 7 gigawatts, 0.2% of the global total, or equivalent to that of Switzerland.

According to Politico, even the high-end estimates of bitcoin's total consumption levels amount to only about 6% of the total power consumed by the global banking sector, and even if bitcoin's consumption levels increased 100 fold from today's levels, bitcoin's consumption would still only amount to about 2% of global power consumption.

To lower the costs, bitcoin miners have set up in places like Iceland where geothermal energy is cheap and cooling Arctic air is free. Bitcoin miners are known to use hydroelectric power in Tibet, Quebec, Washington (state), and Austria to reduce electricity costs. Miners are attracted to suppliers such as Hydro Quebec that have energy surpluses. According to a University of Cambridge study, much of bitcoin mining is done in China, where electricity is subsidized by the government.

Carbon footprint
Concerns about bitcoin's environmental impact relate bitcoin's energy consumption to carbon emissions. The difficulty of translating the energy consumption into carbon emissions lies in the decentralized nature of bitcoin impeding the localization of miners to examine the electricity mix used. The results of recent studies analyzing bitcoin's carbon footprint vary. A study published in Nature Climate Change in 2018 claims that bitcoin "could alone produce enough CO
2 emissions to push warming above 2 °C within less than three decades." However, other researchers criticized this analysis, arguing the underlying scenarios were inadequate, leading to overestimations. According to studies published in Joule and American Chemical Society in 2019, bitcoin's annual energy consumption results in annual carbon emission ranging from 17 to 22.9 MtCO
2 which is comparable to the level of emissions of countries as Jordan and Sri Lanka or Kansas City. International Energy Agency estimates bitcoin-related annual carbon emissions to be likely in a range from 10 to 20 MtCO
2 and characterizes the predictions in Nature Climate Change as just "sensational predictions about bitcoin" echoing the warnings from late 1990s about Internet and its increasing energy consumption.

Ponzi scheme and pyramid scheme concerns
Journalists, economists, investors, and the central bank of Estonia have voiced concerns that bitcoin is a Ponzi scheme. In April 2013, Eric Posner, a law professor at the University of Chicago, stated that "a real Ponzi scheme takes fraud; bitcoin, by contrast, seems more like a collective delusion." A July 2014 report by the World Bank concluded that bitcoin was not a deliberate Ponzi scheme.:7 In June 2014, the Swiss Federal Council:21 examined the concerns that bitcoin might be a pyramid scheme; it concluded that, "Since in the case of bitcoin the typical promises of profits are lacking, it cannot be assumed that bitcoin is a pyramid scheme."

Security issues
Bitcoin is vulnerable to theft through phishing, scamming, and hacking. As of December 2017, around 980,000 bitcoins have been stolen from cryptocurrency exchanges.

Use in illegal transactions
See also: Bitcoin network § Alleged criminal activity
The use of bitcoin by criminals has attracted the attention of financial regulators, legislative bodies, law enforcement, and the media. Bitcoin gained early notoriety for its use on the Silk Road. The U.S. Senate held a hearing on virtual currencies in November 2013. The U.S. government claimed that bitcoin was used to facilitate payments related to Russian interference in the 2016 United States elections.

Several news outlets have asserted that the popularity of bitcoins hinges on the ability to use them to purchase illegal goods. Nobel-prize winning economist Joseph Stiglitz says that bitcoin's anonymity encourages money laundering and other crimes.

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". Australian researchers have estimated that 25% of all bitcoin users and 44% of all bitcoin transactions are associated with illegal activity as of April 2017. There were an estimated 24 million bitcoin users primarily using bitcoin for illegal activity. They held $8 billion worth of bitcoin, and made 36 million transactions valued at $72 billion.

Other critical opinions
François R. Velde, Senior Economist at the Chicago Fed, described it as "an elegant solution to the problem of creating a digital currency".

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 other central banks, because it prompts these institutions to operate sound policies.:33

PayPal President David A. Marcus calls bitcoin a "great place to put assets".

Implementation in software
Bitcoin Core is free and open-source software that serves as a bitcoin node (the set of which form the bitcoin network) and provides a bitcoin wallet which fully verifies payments. It is considered to be bitcoin's reference implementation. Initially, the software was published by Satoshi Nakamoto under the name "Bitcoin", and later renamed to "Bitcoin Core" to distinguish it from the network. It is also known as the Satoshi client.

The MIT Digital Currency Initiative funds some of the development of Bitcoin Core. The project also maintains the cryptography library libse*****256k1.

Features
Bitcoin Core includes a transaction verification engine and connects to the bitcoin network as a full node. Moreover, a cryptocurrency wallet, which can be used to transfer funds, is included by default. The wallet allows for the sending and receiving of bitcoins. It does not facilitate the buying or selling of bitcoin. It allows users to generate QR codes to receive payment.

The software validates the entire blockchain, which includes all bitcoin transactions ever. This distributed ledger which has reached more than 235 gigabytes in size as of Jan 2019, must be downloaded or synchronized before full participation of the client may occur. Although the complete blockchain is not needed all at once since it is possible to run in pruning mode. A command line-based daemon with a JSON-RPC interface, bitcoind, is bundled with Bitcoin Core. It also provides access to testnet, a global testing environment that imitates the bitcoin main network using an alternative blockchain where valueless "test bitcoins" are used. Regtest or Regression Test Mode creates a private blockchain which is used as a local testing environment. Finally, bitcoin-cli, a simple program which allows users to send RPC commands to bitcoind, is also included.

Checkpoints which have been hard coded into the client are used only to prevent Denial of Service attacks against nodes which are initially syncing the chain. For this reason the checkpoints included are only as of several years ago.[failed verification] A one megabyte block size limit was added in 2010 by Satoshi Nakamoto. This limited the maximum network capacity to about three transactions per second. Since then, network capacity has been improved incrementally both through block size increases and improved wallet behavior. A network alert system was included by Satoshi Nakamoto as a way of informing users of important news regarding bitcoin. In November 2016 it was retired. It had become obsolete as news on bitcoin is now widely disseminated.

Bitcoin Core includes a scripting language inspired by Forth that can define transactions and specify parameters. ScriptPubKey is used to "lock" transactions based on a set of future conditions. scriptSig is used to meet these conditions or "unlock" a transaction. Operations on the data are performed by various OP_Codes. Two stacks are used - main and alt. Looping is forbidden.

Bitcoin Core uses OpenTimestamps to timestamp merge commits.

Development
The original creator of the bitcoin client has described their approach to the software's authorship as it being written first to prove to themselves that the concept of purely peer-to-peer electronic cash was valid and that a paper with solutions could be written. The lead developer is Wladimir J. van der Laan, who took over the role on 8 April 2014. Gavin Andresen was the former lead maintainer for the software client. Andresen left the role of lead developer for bitcoin to work on the strategic development of its technology. Bitcoin Core in 2015 was central to a dispute with Bitcoin XT, a competing client that sought to increase the blocksize. Over a dozen different companies and industry groups fund the development of Bitcoin Core.