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Any time a transaction is made, it immediately starts out labeled as ''unconfirmed''. The confirmation status is reflective of the likelihood that the transaction could be successfully reversed in the event of a deliberate attempt to do so. Any transaction broadcast to other nodes does not become ''confirmed'' until acknowledged in a collectively maintained timestamped-list of all known transactions, the ''block chain''.
Any time a transaction is made, it immediately starts out labeled as ''unconfirmed''. The confirmation status is reflective of the likelihood that the transaction could be successfully reversed in the event of a deliberate attempt to do so. Any transaction broadcast to other nodes does not become ''confirmed'' until acknowledged in a collectively maintained timestamped-list of all known transactions, the ''block chain''.


In particular, each ''generating'' node collects all unacknowledged transactions it knows of in a file known as a ''block'',<ref>{{cite web |url=https://en.bitcoin.it/wiki/Block_hashing_algorithm |title=Bitcoin documentation on the block hashing algorithm }}</ref> which references all recent transactions as well as the previous valid block known to that node. It then appends a [[Cryptographic nonce|nonce]] value to this block which is essentially an integer number and computes the [[SHA-256]] [[Cryptographic hash function|cryptographic hash]] of the block with the appended nonce value. It looks for a nonce that produces a hash that starts with a given number of zero bits. Because the hash function is not reversible, finding such a nonce is hard and requires on average a predictable amount of [[Brute-force search|repetitious trial and error]]. When a node finds such a solution, it announces it to the rest of the network. Peers receiving the new solved block validate it by checking that the hash really starts with the given number of zero bits before accepting it and adding it to the chain.
In particular, each ''generating'' node collects all unacknowledged transactions it knows of in a file known as a ''block'',<ref>{{cite web |url=https://en.bitcoin.it/wiki/Block_hashing_algorithm |title=Bitcoin documentation on the block hashing algorithm }}</ref> which references all recent transactions as well as the previous valid block known to that node. It then appends a [[Cryptographic nonce|nonce]] value to this block which is essentially an integer number and computes the [[SHA-256]] [[Cryptographic hash function|cryptographic hash]] of the block with the appended nonce value. It looks for a nonce that produces a hash that starts with a given number of zero bits. Because the hash function is not reversible, finding such a nonce is hard and requires on average a predictable amount of [[Brute-force search|repetitious trial and error]]. When a node finds such a solution, it announces it to the rest of the network. Peers receiving the new solved block validate it by computing the hash and checking that it really starts with the given number of zero bits. Then they accept it and add it to the chain.


When a transaction is first acknowledged in a block, it receives one confirmation. The transaction itself is only acknowledged once, but blocks themselves are acknowledged repeatedly as time passes and the chain grows. Each time that first block is acknowledged by future blocks, the transaction is considered to have received another confirmation. After six confirmations, the Bitcoin client switches from showing "unconfirmed" to "confirmed". Although a transaction could be considered "confirmed" after a single confirmation, the client avoids reporting it as such until several further confirmations to ensure that it is overwhelmingly likely that the transactions are part of the main block chain rather than an ''orphaned'' one, and more importantly, practically impossible to reverse.
When a transaction is first acknowledged in a block, it receives one confirmation. The transaction itself is only acknowledged once, but blocks themselves are acknowledged repeatedly as time passes and the chain grows. Each time that first block is acknowledged by future blocks, the transaction is considered to have received another confirmation. After six confirmations, the Bitcoin client switches from showing "unconfirmed" to "confirmed". Although a transaction could be considered "confirmed" after a single confirmation, the client avoids reporting it as such until several further confirmations to ensure that it is overwhelmingly likely that the transactions are part of the main block chain rather than an ''orphaned'' one, and more importantly, practically impossible to reverse.

Revision as of 20:39, 23 August 2011

Bitcoin
Developer(s)Satoshi Nakamoto (inactive, probably a pseudonym), Gavin Andresen, Pieter Wuille, Nils Schneider, Jeff Garzik
Initial releaseFebruary 4, 2009 (2009-02-04) [citation needed]
Preview release
0.3.24 July 8, 2011; 13 years ago (2011-07-08)
Written inC++
Operating systemWindows, GNU/Linux, Mac OS X, FreeBSD
Available inDutch, English, French, Italian, German, Portuguese, Russian and Spanish.
TypeElectronic money
LicenseMIT License (open-source)
Websitewww.bitcoin.org

Bitcoin is a digital currency created in 2009, based mainly on a self-published paper by Satoshi Nakamoto.[1]

Bitcoin enables payments (and micropayments) at a very low cost, and avoids the need for central authorities and issuers, with an average ten minute wait for confirmation[citation needed]. Digitally signed transactions, with one node signing over some amount of the currency to another node, are broadcast to all nodes in a peer-to-peer network. A proof-of-work system is used as a measurement against double-spending and as an initial currency distribution mechanism.

Operation

Bitcoin software running under Windows 7

People interact with bitcoin using a "wallet," which may be either stored on their computer by the bitcoin software or hosted on a third-party website. The wallet shows users their available bitcoin balance, transaction history, and the collection of bitcoin addresses they may use to send and receive bitcoins with other users. Because all transactions are added to the transaction log in the bitcoin block-chain, which is a distributed database formed by all the bitcoin participants, a user's bitcoin software does not need to be running for that user to receive bitcoins.

Bitcoin payments are normally displayed to the receiver near-instantly, but they are initially displayed as unconfirmed, because the bitcoin system cannot yet assure that the transaction is permanent. A transaction may be invalidated due to conflicting transactions (such as the same bitcoins being sent to two different receivers). This may happen if a sender malfunctions, or if a sender intentionally attempts to defraud a receiver. When the bitcoin network processes the transaction, an increasing number of confirmations are added every time the chain containing the transaction is extended. Eventually, the bitcoin software displays the transaction as confirmed.

The process of confirming a transaction is accomplished by solving a computationally difficult proof-of-work problem.[2] The problem is based on data from the transactions that must be confirmed, as well as the entire previous transaction history. This process makes it infeasible for an attacker to rewrite the transaction history without having more computing power than the rest of the bitcoin system. Nodes that process blocks of transactions are rewarded by receiving a programmed amount of bitcoin, which arises "out of thin air," as well as any transaction fees associated with the transactions they process. This compensates the operators of these systems for their computational work used to secure bitcoin transactions against reversal, and also accomplishes the initial wealth distribution for the bitcoin system as a whole. The difficulty of the proof-of-work problems is automatically adjusted by the system so that the average time between new blocks being awarded is ten minutes. All participating systems check the validity of every transaction and of every block and ignore any that violate the rules, such as blocks that bring the wrong amount of new bitcoin into existence, or transactions that would involve one sender spending the same bitcoin twice.

Economics

As of August 2011, there are over 7.1 million bitcoins in existence.[3] This figure is algorithmically determined as described in Nakamoto's whitepaper. Because by definition the only spendable Bitcoins in existence are those represented in the block chain database passed around on the peer to peer network, the number is not only easy to determine, but can be quickly determined with precision by all participants.[1]

Anyone can view the block-chain and observe transactions in real-time.[4] Currency exchanges also exist between bitcoins and other virtual currencies, such as the Linden Dollar.[5]

Monetary differences

Expected future bitcoin supply (for 10-minute issuance frequency).

Unlike conventional fiat currency, Bitcoin has no centralized issuing authority.[6][7][8] There is a limited controlled expansion of the monetary base hardcoded in the Bitcoin software.[citation needed]

Transfers are facilitated directly without the use of a centralized financial processor between nodes. This type of transaction makes chargebacks unlikely.[9] Bitcoin transactions can represent many kinds of operations such as pure peer-to-peer escrow and deposits but user interface software for this advanced functionality is currently underdeveloped.[citation needed] The Bitcoin client broadcasts transactions to surrounding nodes, which propagate them across the network. Corrupted or invalid transactions are rejected by legitimate clients. Transactions are free; however, an optional, often necessary, fee may be paid to other nodes to prioritize transaction processing.[citation needed]

The total number of bitcoins is programmed to approach 21 million over time.[7] The money supply is programmed to grow as a geometric series every 210,000 blocks (roughly every 4 years); by 2013 half of the total supply will be generated, and by 2017, 3/4 will be generated. To ensure sufficient granularity of the money supply, bitcoins are divisible down to eight decimal places (a total of 2.1 × 1015 or 2.1 quadrillion units).[10]

The diminishing geometric expansion combined with the expansion of Bitcoin users provided an incentive for early adopters, who obtained bitcoin at preferential exchange rates.

Bitcoin's design allows for pseudonymous ownership and transfers. Because of this, Bitcoin has anonymity properties weaker than cash but stronger than traditional electronic payment systems. Although the complete history of every bitcoin transaction is public, it is not possible in general [11] to associate bitcoin identities with real-life identities. This property makes bitcoin transactions attractive to some sellers of illegal products.[12][13]

Outcome

Proposed failure scenarios for Bitcoin include a declining user base, the discovery of a flaw in the code, the advent of a computing power sufficient to bruteforce the cryptography used, or a global governmental crackdown on the software and exchanges. Succession to another similar crypto-currency system is also possible, if a new one were created and considered more legitimate or advantageous than Bitcoin in its current form (e.g., more scalable or user-friendly). It may not be possible to "ban all crypto-cash like Bitcoin".[14] In an Irish Times investigative article Danny O'Brien reported "When I show people this Bitcoin economy, they ask: 'Is this legal?' They ask: 'Is it a con?' I imagine there are lawyers and economists struggling to answer both questions. I suspect you will be able to add lawmakers to that list shortly."[14]

U.S. Senators have written a letter to Attorney General Eric Holder and the chief of the DEA regarding the use of Bitcoin in "black market" transactions.[15] Consequently Amir Taaki from Britcoin, the UK exchange, put out a statement calling for regulation of Bitcoin exchanges by law enforcement.[16][17]

Technology

The principles of the system are described in Satoshi Nakamoto's 2008 Bitcoin whitepaper,[1] which was posted to a cryptography e-mail list.[18] Bitcoin relies on the transfer of amounts between public accounts using digital signatures. All transactions are public and stored in a distributed database that is used to confirm transactions and prevent double-spending.

Addresses

Bitcoin is based on public-key cryptography. Any person participating in the Bitcoin network has a wallet containing an arbitrary number of cryptographic keypairs. The user's public keys are transformed into Bitcoin addresses, which act as the receiving endpoints for all payments. The corresponding private keys are needed to authorize payments from that user's wallet. Addresses contain no information about their owner although owners may be traceable through the distributed transaction history. Addresses in human-readable form are strings of random numbers and letters around 33 characters in length, always beginning with the number 1, as in the example of 175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W.[19]

Transactions

Bitcoins contain the current owner's wallet address. A user can create as many wallets as they wish. When a bitcoin belonging to user A is transferred to user B, then A’s ownership over that bitcoin is relinquished by adding B’s address to it and signing the result with the private key that is associated with A’s address.[20] Because of the asymmetric cryptographic method, nobody else can grant this signature, and the private key cannot be determined based on the signed bitcoin.[Notes 1] The resulting bitcoin is broadcast in a message, the transaction, on the peer-to-peer network. The rest of the network nodes validate the cryptographic signatures and the amounts of the transaction before accepting it.[7]

Because transactions are broadcast to the entire network, they are inherently public. Unlike regular banking, which preserves customer privacy by keeping transaction records private, transactional anonymity is accomplished in Bitcoin by keeping the ownership of addresses private, while at the same time publishing all transactions. As an example, if Alice sends 123.45 BTC to Bob, a public record is created that allows anyone to see that 123.45 was sent from one address to another. However, unless Alice or Bob make their ownership of these addresses publicly known in some way, it is difficult for anyone else to connect the transaction with them. However, if an address is connected to a user at any point it can be possible to follow back a series of transactions because each participant likely knows who paid them and may disclose that information on request or under duress. Bitcoin thus provides anonymity that is weaker than cash transactions but stronger than other popular electronic transactions.[21][citation needed]

Block-chain and confirmations

The main chain (black) consists of the longest series of blocks from the genesis block (green) to the current block. Orphan blocks (grey) exist outside of the main chain.

To prevent double-spending, the network implements what Nakamoto describes as a peer-to-peer distributed timestamp server, which assigns sequential identifiers to each transaction, which are then hardened against modification using the idea of chained proofs of work (shown in the Bitcoin client as confirmations). In his white paper, Nakamoto wrote: "we propose a solution to the double-spending problem using a peer-to-peer distributed timestamp server to generate computational proof of the chronological order of transactions."[1]

Any time a transaction is made, it immediately starts out labeled as unconfirmed. The confirmation status is reflective of the likelihood that the transaction could be successfully reversed in the event of a deliberate attempt to do so. Any transaction broadcast to other nodes does not become confirmed until acknowledged in a collectively maintained timestamped-list of all known transactions, the block chain.

In particular, each generating node collects all unacknowledged transactions it knows of in a file known as a block,[22] which references all recent transactions as well as the previous valid block known to that node. It then appends a nonce value to this block which is essentially an integer number and computes the SHA-256 cryptographic hash of the block with the appended nonce value. It looks for a nonce that produces a hash that starts with a given number of zero bits. Because the hash function is not reversible, finding such a nonce is hard and requires on average a predictable amount of repetitious trial and error. When a node finds such a solution, it announces it to the rest of the network. Peers receiving the new solved block validate it by computing the hash and checking that it really starts with the given number of zero bits. Then they accept it and add it to the chain.

When a transaction is first acknowledged in a block, it receives one confirmation. The transaction itself is only acknowledged once, but blocks themselves are acknowledged repeatedly as time passes and the chain grows. Each time that first block is acknowledged by future blocks, the transaction is considered to have received another confirmation. After six confirmations, the Bitcoin client switches from showing "unconfirmed" to "confirmed". Although a transaction could be considered "confirmed" after a single confirmation, the client avoids reporting it as such until several further confirmations to ensure that it is overwhelmingly likely that the transactions are part of the main block chain rather than an orphaned one, and more importantly, practically impossible to reverse.

Eventually, the block-chain contains the cryptographic ownership history of all coins from their creator-address to their current owner-address.[23] Therefore, if a user attempts to reuse coins he already spent, the network rejects the transaction.

The whole history of transactions must be stored inside the block chain database, which grows constantly as new records are added and never removed. By design, some but not all users need the entire database to use Bitcoin – some users only need the portion of the database that pertains to the coins they own or might receive in the future. Presently, the database is manageable enough (less than 600 MB as of July 2011) that all users of the Bitcoin software receive the entire database over the peer-to-peer network after running the software the first time.

Nakamoto conceived that as the database became larger, applications for Bitcoin without the entire database on each user's computer would be desirable. To enable this, a Merkle tree is used to organize the transaction records in such a way that a future Bitcoin client can locally delete portions of its own database it knows it will never need, such as earlier transaction records of bitcoins that have changed ownership multiple times, while keeping the cryptographic integrity of the remaining database intact.

Generation

To make sybil attacks against the block-chain consensus practically infeasible, the generation of a Bitcoin block requires finding the solution to a difficult cryptographic proof-of-work problem. Nodes that attempt to generate blocks are called "miners." They repeatedly try solving instances of the problem through trial and error, each attempt having an equal and very low prior chance of being a correct solution. The probability of success is adjusted automatically by the protocol[citation needed] in steps after 2016 blocks have been created to regulate the rate of new block creation. As a result, the rate at which a given user solves blocks depends on the computing power that user contributes to the network relative to the computing power of all nodes combined.[24] All newly announced blocks are validated by all Bitcoin nodes to ensure that they conform to the protocol rules before they are accepted, added to the block chain, and forwarded on.

Because block solutions arise out of an independent random process, block creation by the Bitcoin network can be described as a Poisson process. The protocol adjusts the problem difficulty so that the distribution mean is λ = 2016 blocks per two weeks, so there are roughly ten minutes between the creation of new blocks on average (the wait times between events in a Poisson process follow an exponential distribution). The difficulty updates happen every 2016 blocks. The difficulty is set to the value that would have most likely caused the prior 2016 blocks to take two weeks to complete, given the same computational effort (according to the timestamps recorded in the blocks).[25] All nodes perform and enforce the same difficulty calculation.

"Mining"

In addition to the pending transactions confirmed in the block, a generating node adds a "generate" transaction, which awards new Bitcoins to the operator of the node that generated the block. The payout of this generated transaction is set according to the inflation schedule programmed into the protocol. The process of solving blocks is often referred to as mining, as in gold mining, in reference to the coins brought into existence by the generate transactions.[1] The "miner" that generates a block also receives the surplus from any transactions that have input value in excess of the output value, effectively a transaction fee that provides an incentive to give a transaction priority for faster confirmation.

The proof-of-work problems are especially suitable to GPUs and specialized hardware. Because of the growing computing power behind the system driving the difficulty to high levels, individual contributors with typical CPUs are no longer likely to solve a block on their own but can still receive part of the Bitcoin generated in a new block by contributing their processing power to a mining pool.[26] This increased difficulty makes it cost prohibitive for an attacker to perform double-spending attacks so it is beneficial to the system.

The number of Bitcoins created per block is never more than 50 BTC, and the awards are programmed to decrease over time towards zero, such that no more than 21 million will ever exist.[10] As this payout decreases, the motive for users to run block-generating nodes is expected to change to earning transaction fees, funding from supporting auxiliary block-chains,[27] and simply to improve the security of the public Bitcoin infrastructure they depend on.

Transaction fees

Miners have no obligation to include transactions in the blocks they try to solve. A transaction fee can be associated with any transaction, giving miners an incentive to put the transaction in a block, as miners collect the transaction fees associated with all transactions included in blocks they solve. Very small transactions, or those that use relatively new coins, have low "priority" and may be assessed a transaction fee to reduce spam. As of version 0.3.23 of the official bitcoin client, the minimum transaction fee for low priority transactions is 0.0005 BTC.

Alternative implementations

Besides the original C++ Bitcoin client, there is an open source implementation of the Bitcoin protocol in Java called BitCoinJ.[28]

Alternative user interfaces include bitcoin-js-remote, a JavaScript web user interface for Bitcoin wallets,[29] as well as Spesmilo, a PySide interface more open to a diversity of users, which can run independently of an external wallet.[30]

Anonymity

Some question just how anonymous bitcoins are.[31]

In an interview with the press, Jeff Garzik, one of the Bitcoin developers, wrote that all Bitcoin transactions are recorded in a public log, though the identities of all the parties are anonymous. He concluded that "attempting major illicit transactions with bitcoin, given existing statistical analysis techniques deployed in the field by law enforcement, is pretty damned dumb".[32][33]. He also said "We are working with the government to make sure indeed the long arm of the government can reach Bitcoin... the only way bit coins are gonna be successful is working with regulation and with the government"[34]

Conversion to and from other currencies

Conversion to and from other currencies can be done in person at local exchangers, but is more commonly performed online through sites such as Mt. Gox, Camp BX, Bitomat, Britcoin, and TradeHill exchange services.[35] As of July 2011, Mt. Gox handles over 80% of all Bitcoin trade volume.[36] Camp BX is a new entrant in Bitcoin trading market, which has made a serious attempt to legitimize Bitcoin trading in the United States.[37]

After two US Senators drew attention to Bitcoin for being the currency used in the Silk Road anonymous marketplace which is portrayed in the media as a drug market place, Reuters wrote that "the Justice Department and other law enforcement agencies may be able to target" entities like Mt. Gox.[38] Consequently Amir Taaki from Britcoin, the UK exchange, put out a statement calling for regulation of Bitcoin exchanges by law enforcement.[16][17]

Creator

Bitcoin was created by a person or persons going by the name Satoshi Nakamoto. Nakamoto self-published a white paper on Bitcoin in 2008 on the cryptography mailing list[39] and then in 2009 founded the open source project called Bitcoin. The real identity of "Satoshi Nakamoto" is unknown. In his P2P Foundation profile[40] he said he is from Japan.

Significant events

  • During early June 2011, there have been media reports of even more individuals purchasing "mining rigs", hardware-customized computers specifically dedicated to generate bitcoin hashes based on GPU, following the rise in Bitcoin prices in the past months.[41]
  • On 19 June 2011, Bitcoin prices against the USD remained high, despite a security breach of the Mt. Gox Bitcoin Exchange, which notably caused the leaking of usernames, emails and MD5 hashed passwords of over 60,000 users onto the Web. A majority of those passwords were also salted. Passwords that were not salted had not been used recently. The price of a Bitcoin briefly dropped to $0.01 on the Mt. Gox exchange (but remained unaffected on other exchanges) after a hacker using credentials from a Mt. Gox auditor's compromised computer illegally transferred a large number of Bitcoins to himself and sold them all, creating a massive "ask" order at any price. Within minutes the price rebounded to over $15 before Mt. Gox shut down their exchange and canceled all trades that happened during the hacking period.[42] The exchange rate of Bitcoins quickly returned to near pre-crash values.[43][44][45][46] This incident sparked a new breed of Bitcoin trading websites claiming better security.[47][48]

Adoption

Wikileaks,[49][50][51] Freenet,[52] Pioneer One,[53] and several others[54] already accept donations in Bitcoin. The Electronic Frontier Foundation did for a while but stopped doing so because of lack of legal precedent about new currency systems, and because they "generally don't endorse any type of product or service – and Bitcoin is no exception."[55] Gavin Andresen, one of the "core developers", is explicitly advising people "not to make heavy investments in Bitcoins", as it is "kind of like a high risk investment".[56] As of July 2011, no publicly traded retailer accepts the currency for payment, while many small businesses have started to do it[57]. A frequent problem faced by retailers willing to accept Bitcoin is the big volatility of its exchange rate to the US dollar and the absence of futures and options permitting to hedge this volatility yet.

Criticism

Unfair initial distribution

A common[58][citation needed] criticism is that the initial bitcoin distribution is heavily advantageous towards early-adopters. As stated, bitcoins are distributed ("generated") as a reward for the solution to a difficult proof-of-work problem. The drawback is that the amount of work that must be done for one bitcoin is currently over 500,000 times more than the amount of work at which the first bitcoins were going. As more people join, and also because of a reward function that halves the number of rewarded bitcoins every so many blocks, it becomes harder to generate bitcoins over time, using the same computing power.[59][60] Because early adopters now have a disproportionate amount of Bitcoins compared to newcomers and that newcomers will never have a chance to "catch up", the current hype being generated by proponents has also been labeled by critics as a pump and dump scheme by early adopters looking to cash in on their large Bitcoin collections.[61]

Technical complexity

Transactions are based on scripts contained in the transactions that must be executed to "access the money." While this increases flexibility of transactions, it does complicate the development of Bitcoin applications.[62][citation needed]

Fluctuation

As the value of a bitcoin is not yet sticky, prices fluctuate relative to goods more than more widely accepted currencies.[63] A further complication is that there is not a single price at a given time, but many prices on different exchanges.[64]

Malware and theft

A bitcoin account can be compromised if a computer with a wallet file can be remotely accessed by hackers or becomes infected by a virus or trojan. The first malware specifically targeting Bitcoin wallets was discovered June 16, 2011.[65][66][67][68] Current Bitcoin clients lack functionality for encrypting the wallet, however, the current development tree contains this feature and it will be available in the next release.[69]

Criminal uses

Bitcoin is the currency used by the Silk Road online marketplace.[32][70] In a 2011 letter to Attorney General Eric Holder and the Drug Enforcement Administration, Senators Charles Schumer of New York and Joe Manchin of West Virginia called for an investigation into Bitcoin and Silk Road.[70] Schumer described the use of Bitcoins at Silk Road as a form of money laundering.[7] Consequently Amir Taaki from Britcoin, the UK exchange, put out a statement calling for regulation of Bitcoin exchanges by law enforcement.[16][17] The hacking organization "LulzSec" accepted donations in Bitcoin, having said that the group "needs bitcoin donations to continue their hacking efforts".[15][71]

Covert "mining"

In June 2011, Symantec warned about the possibility of botnets engaging in covert "mining" of bitcoins (unauthorized use of computer resources to mount brute force attacks against bitcoins).[72][73], consuming computing cycles, using extra electricity and possibly increasing the temperature of the computer. Later that month, an employee of the Australian Broadcasting Corporation was caught after using the company's servers to generate bitcoins without permission.[74] Bitcoin mining can be sped up considerably by using the parallel processing capabilities of the GPUs built into many modern-day video cards.[75]

See also

Notes

  1. ^ This statement is made on the assumption that it is infeasible to brute force the keys, and that no side channel attack is possible either.

References

  1. ^ a b c d e Nakamoto, Satoshi (24 May 2009), Bitcoin: A Peer-to-Peer Electronic Cash System (PDF), retrieved 14 December 2010
  2. ^ Thomas Lowenthal (8 June 2011). "Bitcoin: inside the encrypted, peer-to-peer digital currency". Ars Technica. Retrieved 14 June 2011.
  3. ^ "Total Number of Bitcoins in Existence". Bitcoin Block Explorer. 23 August 2011. Retrieved 23 August 2011.
  4. ^ "Home – Bitcoin Block Explorer". Retrieved 8 June 2011.
  5. ^ "Bitcoin Mining: The Free Lottery". Radoff.com. Retrieved 3 June 2011.
  6. ^ Sponsored by. "Virtual currency: Bits and bob". The Economist. Retrieved 22 June 2011.
  7. ^ a b c d Lowenthal, Thomas (06-08-11). "Bitcoin: inside the encrypted, peer-to-peer digital currency". Ars Technica. {{cite news}}: Check date values in: |date= (help)
  8. ^ Geere, Duncan. "Peer-to-peer currency Bitcoin sidesteps financial institutions (Wired UK)". Wired.co.uk. Retrieved 22 June 2011.
  9. ^ "Bitcoin documentation on chargebacks".
  10. ^ a b Nathan Willis (10 November 2010). "Bitcoin: Virtual money created by CPU cycles". LWN.net.
  11. ^ Fergal Reid and Martin Harrigan (July 24th 2011). An Analysis of Anonymity in the Bitcoin System. An Analysis of Anonymity in the Bitcoin System.
  12. ^ Andy Greenberg (April 20, 2011). Crypto Currency. Forbes Magazine.
  13. ^ Madrigal, Alexis (1 June 2011). "Libertarian Dream? A Site Where You Buy Drugs With Digital Dollars". The Atlantic Monthly. Retrieved 5 June 2011.
  14. ^ a b O'Brien, Danny (26 November 2010). "Imagine your computer as a wallet full of Bitcoins". The Irish Times. Retrieved 19 December 2010.
  15. ^ a b Reisinger, Don (9 June 2011). "Senators target Bitcoin currency, citing drug sales | The Digital Home – CNET News". News.cnet.com. Retrieved 22 June 2011.
  16. ^ a b c Reuters – Bitcoin exchanges offer anti- money-laundering aid
  17. ^ a b c Britcoin.co.uk – statement to reuters from bitomat.pl and Britcoin.co.uk
  18. ^ Tom Simonite (25 May 2011). "What Bitcoin Is, and Why It Matters". Technology Review. MIT. Retrieved 14 June 2011.
  19. ^ "Bitcoin documentation on addresses".
  20. ^ "Transactions - Bitcoin". En.bitcoin.it. 16 June 2011. Retrieved 22 June 2011.
  21. ^ "Bitcoin documentation on anonymity".
  22. ^ "Bitcoin documentation on the block hashing algorithm".
  23. ^ "Bitcoin Block Explorer".
  24. ^ Luongo, Thomas (23 July 2010). "The FED's Real Monetary Problem". LewRockwell.com. Retrieved 12 October 2010.
  25. ^ "Bitcoin documentation on difficulty". Retrieved 26 May 2011.
  26. ^ "Bitcoin Wiki: Pooled mining". Retrieved 26 May 2011.
  27. ^ "Bitcoin forum thread on auxiliary block chains". Retrieved 26 May 2011.
  28. ^ angry tapir, timothy (23 March 2011). "Google Engineer Releases Open Source Bitcoin Client". Slashdot. Retrieved 18 May 2011.
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  30. ^ "Spesmilo, PySide Bitcoin client". En.bitcoin.it. 19 May 2011. Retrieved 22 June 2011.
  31. ^ "Bitcoin not so anonymous, Irish researcher says – Cyrus Farivar – Science and Technology". Deutsche Welle. 1 June 2011. Retrieved 29 July 2011.
  32. ^ a b Chen, Adrian (06-01-2011). "The Underground Website Where You Can Buy Any Drug Imaginable". Gawker. {{cite news}}: Check date values in: |date= (help)
  33. ^ "Libertarian Dream? A Site Where You Buy Drugs With Digital Dollars – Alexis Madrigal – Technology". The Atlantic. 1 June 2011. Retrieved 22 June 2011.
  34. ^ "CBSNewsOnline: BitCoin: The Future of Currency".
  35. ^ "How to use bitcoin". bitcoinme.
  36. ^ Bitcoin Charts
  37. ^ Camp BX aims to make Bitcoin trading legit, VentureBeat
  38. ^ Brett Wolf, (Jun 8, 2011) Senators seek crackdown on "Bitcoin" currency, Reuters
  39. ^ Satoshi's posts to Cryptography mailing list
  40. ^ Satoshi Nakamoto profile on P2P Foundation
  41. ^ Timothy McDonald, 9 June 2011, New online currency offers a 'better way', Australian Broadcasting Corporation
  42. ^ MtGox Clarification on June 19 hack
  43. ^ Jason Mick, 19 June 2011, Inside the Mega-Hack of Bitcoin: the Full Story, DailyTech
  44. ^ Timothy B. Lee, 19 June 2011, Bitcoin prices plummet on hacked exchange, Ars Technica
  45. ^ Mark Karpeles, 20 June 2011, Huge Bitcoin sell off due to a compromised account – rollback, Mt.Gox Support
  46. ^ Chirgwin, Richard (19 June 2011). "Bitcoin collapses on malicious trade – Mt Gox scrambling to raise the Titanic". The Register.
  47. ^ Forum Announcement, June 28, 2011, Security Audit Results for Camp BX, Camp BX Security Audit - Public Review
  48. ^ VentureBeat, July 07 2011, VentureBeat - Making Bitcoin Trading Legit, VentureBeat - Making Bitcoin Trading Legit
  49. ^ "Donate". Wikileaks.org. Retrieved 22 June 2011.
  50. ^ http://twitter.com/#!/wikileaks/status/80774521350668288
  51. ^ Greenberg, Andy (14 June 2011). "WikiLeaks Asks For Anonymous Bitcoin Donations - Andy Greenberg - The Firewall - Forbes". Blogs.forbes.com. Retrieved 22 June 2011.
  52. ^ "/donate". The Freenet Project. Retrieved 22 June 2011.
  53. ^ http://twitter.com/#!/pioneeronetv/status/36119594439544832
  54. ^ "Donation-accepting organizations and projects – Bitcoin". En.bitcoin.it. Retrieved 22 June 2011.
  55. ^ "EFF and Bitcoin | Electronic Frontier Foundation". Eff.org. 14 June 2011. Retrieved 22 June 2011.
  56. ^ "/59/Bitcoin – a Digital, Decentralized Currency (at 31 min.)". omega tau podcast. 19 March 2011.
  57. ^ https://en.bitcoin.it/wiki/Trade
  58. ^ http://www.google.com/search?q=bitcoin+early+adopter+unfair
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