Bitcoin Wiki - User contributions [en]

Help:FAQ

2013-06-27T11:08:15Z

Oconnor663: Mars thought experiment

Here you will find answers to the most commonly asked questions.

== General ==
=== What are bitcoins? ===
Bitcoins are the unit of currency of the Bitcoin system. A commonly used shorthand for this is “BTC” to refer to a price or amount (eg: “100 BTC”).
There are such things as [[physical bitcoins]], but ultimately, a bitcoin is just a number associated with a [[Address|Bitcoin Address]]. A physical bitcoin is simply an object, such as a coin, with the number carefully embedded inside. See also an [[Introduction|easy intro]] to bitcoin.

=== How can I get bitcoins? ===

There are a variety of ways to acquire bitcoins:

* Accept bitcoins as payment for goods or services.
* The most common way to buy bitcoins are the [[Buying bitcoins|Bitcoin Exchanges]]
* There are several services where you can [[Buying_Bitcoins_(the_noob_version)|trade them]] for traditional currency.
* Find someone to trade cash for bitcoins in-person through a [https://en.bitcoin.it/wiki/Category:Directories local directory].
* Participate in a [[Pooled mining|mining pool]].
* If you have a lot of mining hardware, you can solo mine and attempt to create a new [[block]] (currently yields 25 bitcoins plus transaction fees).
* Visit sites that provide [[Trade#Free_Samples_and_Offers|free samples and offers]].

===Does Bitcoin guarantee an influx of free money?===

Since Bitcoin is a new technology, what it is and how it works may be initially unclear. Bitcoin is sometimes presented as being one of three things:
<ol style="list-style-type: upper-alpha;">
<li>Some sort of online 'get-rich-quick' scam.</li>
<li>A loophole in the market economy, the installation of which guarantees a steady influx of cash.</li>
<li>A sure investment that will almost certainly yield a profit.</li>
</ol>
In fact, none of the above are true. Let's look at them independently.

;Is Bitcoin a 'get-rich-quick' scheme?
:If you've spent much time on the Internet, you've probably seen ads for many 'get-rich-quick' schemes. These ads usually promise huge profits for a small amounts of easy work. Such schemes are usually pyramid/matrix-style schemes that make money from their own employees and offer nothing of any real value. Most convince one to buy packages that will make them earn hundreds a day, which in fact have the buyer distribute more such ads, and make minute profits.

:Bitcoin is in no way similar to these schemes. Bitcoin doesn't promise windfall profits. There is no way for the developers to make money from your involvement or to take money from you. That bitcoins are nearly impossible to acquire without the owner's consent represents one of its greatest strengths. Bitcoin is an experimental, virtual currency that may succeed or may fail. None of its developers expect to get rich off of it.

:A more detailed answer to this question can be found [http://bitcointalk.org/?topic=7815.0 here].

;Will I make money by installing the client?
:Most people who use Bitcoin don't earn anything by doing so, and the default client has no built-in way to earn Bitcoins. A small minority of people with dedicated, high-performance hardware do earn some Bitcoins by "''mining''" (generating new bitcoins, see [[#What is mining?|What is mining?]]) with special software, but joining Bitcoin shouldn't be construed as being the road to riches. Most Bitcoin users get involved because they find the project conceptually interesting and don't earn anything by doing so. This is also why you won't find much speculation about the political or economic repercussions of Bitcoin anywhere on this site: Bitcoin developers owe their dedication to the project's intellectual yieldings more than to those of a monetary nature. Bitcoin is still taking its first baby steps; it may go on to do great things but right now it only has something to offer those chasing conceptually interesting projects or bleeding edge technology.

;As an investment, is Bitcoin a sure thing?
:Bitcoin is a new and interesting electronic currency, the value of which is not backed by any single government or organization. Like other currencies, it is worth something partly because people are willing to trade it for goods and services. Its exchange rate fluctuates continuously, and sometimes wildly. It lacks wide acceptance and is vulnerable to manipulation by parties with modest funding. Security incidents such as website and account compromise may trigger major sell-offs. Other fluctuations can build into positive feedback loops and cause much larger exchange rate fluctuations. Anyone who puts money into Bitcoin should understand the risk they are taking and consider it a high-risk currency. Later, as Bitcoin becomes better known and more widely accepted, it may stabilize, but for the time being it is unpredictable. Any investment in Bitcoin should be done carefully and with a clear plan to manage the risk.

=== Can I buy bitcoins with Paypal? ===

It is possible to buy [[physical bitcoins]] with PayPal but it is otherwise difficult and/or expensive to do so for non-physical bitcoins, because of significant risk to the seller.

While it is possible to find an individual who wishes to sell Bitcoin to you via Paypal, (perhaps via [http://www.bitcoin-otc.com/ #bitcoin-otc] ) most exchanges do not allow funding through PayPal. This is due to repeated cases where someone pays for bitcoins with Paypal, receives their bitcoins, and then fraudulently complains to Paypal that they never received their purchase. PayPal often sides with the fraudulent buyer in this case, which means any seller needs to cover that risk with higher fees or refuse to accept PayPal altogether.

Buying Bitcoins from individuals this way is still possible, but requires the seller to have some trust that the buyer will not file a claim with PayPal to reverse the payment.

=== Where can I find a forum to discuss Bitcoin? ===

Please visit the [[Bitcoin:Community_portal#Bitcoin_Community_Forums_on_various_platforms|Community Portal]] for links to Bitcoin-related forums.

=== How are new bitcoins created? ===

[[File:total_bitcoins_over_time_graph.png|thumb|Number of bitcoins over time, assuming a perfect 10-minute interval.]]
New bitcoins are generated by the network through the process of "[[#What is mining?|''mining'']]". In a process that is similar to a continuous raffle draw, mining nodes on the network are awarded bitcoins each time they find the solution to a certain mathematical problem (and thereby create a new [[block]]). Creating a block is a [[proof of work]] with a difficulty that varies with the overall strength of the network. The reward for solving a block is [[Controlled Currency Supply|automatically adjusted]] so that roughly every four years of operation of the Bitcoin network, half the amount of bitcoins created in the prior 4 years are created. {{formatnum:10500000}} bitcoins were created in the first 4 (approx.) years from January 2009 to November 2012. Every four years thereafter this amount halves, so it will be {{formatnum:5250000}} over years 4-8, {{formatnum:2625000}} over years 8-12, and so on. Thus the total number of bitcoins in existence will never exceed {{formatnum:21000000}}. See [[Controlled Currency Supply]].

Blocks are [[Mining|mined]] every 10 minutes, on average and for the first four years ({{formatnum:210000}} blocks) each block included 50 new bitcoins. As the amount of processing power directed at mining changes, the difficulty of creating new bitcoins changes. This difficulty factor is calculated every 2016 blocks and is based upon the time taken to generate the previous 2016 blocks. See [[Mining]].

=== What's the current total number of bitcoins in existence? ===

[http://blockexplorer.com/q/totalbc Current count]. Also see [https://blockchain.info/charts/total-bitcoins Total bitcoins in circulation chart]

The number of blocks times the coin value of a block is the number of coins in existence. The coin value of a block is 50 BTC for each of the first {{formatnum:210000}} blocks, 25 BTC for the next {{formatnum:210000}} blocks, then 12.5 BTC, 6.25 BTC and so on.

=== How divisible are bitcoins? ===

A bitcoin can be divided down to 8 decimal places. Therefore, 0.00000001 BTC is the smallest amount that can be handled in a transaction. If necessary, the protocol and related software can be modified to handle even smaller amounts.

=== What do I call the various denominations of bitcoin? ===

There is a lot of discussion about the naming of these fractions of bitcoins. The leading candidates are:

* 1 BTC = 1 bitcoin
* 0.01 BTC = 1 cBTC = 1 centibitcoin (also referred to as bitcent)
* 0.001 BTC = 1 mBTC = 1 millibitcoin (also referred to as mbit (pronounced em-bit) or millibit or even bitmill)
* 0.000 001 BTC = 1 μBTC = 1 microbitcoin (also referred to as ubit (pronounced yu-bit) or microbit)

The above follows the accepted international SI prefixes for hundredths, thousandths, and millionths. There are many arguments against the special case of 0.01 BTC since it is unlikely to represent anything meaningful as the Bitcoin economy grows (it certainly won't be the equivalent of 0.01 USD, GBP or EUR). Equally, the inclusion of existing national currency denominations such as "cent", "nickel", "dime", "pence", "pound", "kopek" and so on are to be discouraged; this is a worldwide currency.

One exception is the "satoshi" which is smallest denomination currently possible

* 0.000 000 01 BTC = 1 satoshi (pronounced sa-toh-shee)
which is so named in honour of Satoshi Nakamoto, the pseudonym of the inventor of Bitcoin.

For an overview of all defined units of Bitcoin (including less common and niche units), see [[Units]].

Further discussion on this topic can be found on the forums here:

* [https://bitcointalk.org/index.php?topic=14438.msg195287#msg195287 We need names]
* [https://bitcointalk.org/index.php?topic=8282.0 What to call 0.001 BTC]

=== How does the halving work when the number gets really small? ===

Eventually the reward will go from 0.00000001 BTC to zero and no more bitcoins will be created.

The block reward calculation is done as a right bitwise shift of a 64-bit signed integer, which means it is divided by two and rounded down. The integer is equal to the value in BTC * 100,000,000 since internally in the reference client software, all Bitcoin balances and values are stored as unsigned integers.

With an initial block reward of 50 BTC, it will take many 4-year periods for the block reward to reach zero.

=== How long will it take to generate all the coins? ===

The last block that will generate coins will be block #6,929,999 which should be generated at or near the year 2140. The total number of coins in circulation will then remain static at 20,999,999.9769 BTC.

Even if the allowed precision is expanded from the current 8 decimals, the total BTC in circulation will always be slightly below 21 million (assuming everything else stays the same). For example, with 16 decimals of precision, the end total would be 20,999,999.999999999496 BTC.

=== If no more coins are going to be generated, will more blocks be created? ===

Absolutely! Even before the creation of coins ends, the use of [[transaction fee|transaction fees]] will likely make creating new blocks more valuable from the fees than the new coins being created. When coin generation ends, these fees will sustain the ability to use bitcoins and the Bitcoin network. There is no practical limit on the number of blocks that will be mined in the future.

=== But if no more coins are generated, what happens when Bitcoins are lost? Won't that be a problem? ===

Because of the law of supply and demand, when fewer bitcoins are available the ones that are left will be in higher demand, and therefore will have a higher value. So, as Bitcoins are lost, the remaining bitcoins will eventually increase in value to compensate. As the value of a bitcoin increases, the number of bitcoins required to purchase an item '''de'''creases. This is a [[Deflationary spiral|deflationary economic model]]. As the average transaction size reduces, transactions will probably be denominated in sub-units of a bitcoin such as millibitcoins ("Millies") or microbitcoins ("Mikes").

The Bitcoin protocol uses a base unit of one hundred-millionth of a Bitcoin ("a Satoshi"), but unused bits are available in the protocol fields that could be used to denote even smaller subdivisions.

=== If every transaction is broadcast via the network, does Bitcoin scale? ===
The Bitcoin protocol allows lightweight clients that can use Bitcoin without downloading the entire transaction history. As traffic grows and this becomes more critical, implementations of the concept will be developed. Full network nodes will at some point become a more specialized service.

With some modifications to the software, full Bitcoin nodes could easily keep up with both VISA and MasterCard combined, using only fairly modest hardware (a single high end server by todays standards). It is worth noting that the MasterCard network is structured somewhat like Bitcoin itself - as a peer to peer broadcast network.

Learn more about [[Scalability]].

==Economy==
=== Where does the value of Bitcoin stem from? What backs up Bitcoin? ===
Bitcoins have value because they are useful and because they are [[Controlled Currency Supply|scarce]]. As they are accepted by more merchants, their value will [http://en.wikipedia.org/wiki/Sticky_%28economics%29 stabilize]. See the [[Trade|list of Bitcoin-accepting sites]].

When we say that a currency is backed up by gold, we mean that there's a promise in place that you can exchange the currency for gold. Bitcoins, like dollars and euros, are not backed up by anything except the variety of merchants that accept them.

It's a common misconception that Bitcoins gain their value from the cost of electricity required to generate them. Cost doesn't equal value – hiring 1,000 men to shovel a big hole in the ground may be costly, but not valuable. Also, even though scarcity is a critical requirement for a useful currency, it alone doesn't make anything valuable. For example, your fingerprints are scarce, but that doesn't mean they have any exchange value.

Alternatively it needs to be added that while the law of supply and demand applies it does not guarantee value of Bitcoins in the future. If confidence in Bitcoins is lost then it will not matter that the supply can no longer be increased, the demand will fall off with all holders trying to get rid of their coins. An example of this can be seen in cases of state currencies, in cases when the state in question dissolves and so no new supply of the currency is available (the central authority managing the supply is gone), however the demand for the currency falls sharply because confidence in its purchasing power disappears. Of-course Bitcoins do not have such central authority managing the supply of the coins, but it does not prevent confidence from eroding due to other situations that are not necessarily predictable.

=== Is Bitcoin a bubble? ===
Yes, in the same way as the euro and dollar are. They only have value in exchange and have no inherent value. If everyone suddenly stopped accepting your dollars, euros or bitcoins, the "bubble" would burst and their value would drop to zero. But that is unlikely to happen: even in Somalia, where the government collapsed 20 years ago, [http://en.wikipedia.org/wiki/Somali_shilling Somali shillings] are still accepted as payment.

=== Is Bitcoin a Ponzi scheme? ===
In a Ponzi Scheme, the founders persuade investors that they’ll profit. Bitcoin does not make such a guarantee. There is no central entity, just individuals building an economy.

A ponzi scheme is a zero sum game. Early adopters can only profit at the expense of late adopters. Bitcoin has possible win-win outcomes. Early adopters profit from the rise in value. Late adopters, and indeed, society as a whole, benefit from the usefulness of a stable, fast, inexpensive, and widely accepted p2p currency.

The fact that early adopters benefit more doesn't alone make anything a Ponzi scheme. All good investments in successful companies have this quality.

=== Doesn't Bitcoin unfairly benefit early adopters? ===
Early adopters have a large number of bitcoins now because they took a risk and invested resources in an unproven technology. By so doing, they have helped Bitcoin become what it is now and what it will be in the future (hopefully, a ubiquitous decentralized digital currency). It is only fair they will reap the benefits of their successful investment.

In any case, any bitcoin generated will probably change hands dozens of time as a medium of exchange, so the profit made from the initial distribution will be insignificant compared to the total commerce enabled by Bitcoin.

Since the pricing of Bitcoins has fallen greatly from its June 2011 peak, prices today are much more similar to those enjoyed by many early adopters. Those who are buying Bitcoins today likely believe that Bitcoin will grow significantly in the future. Setting aside the brief opportunity to have sold Bitcoins at the June 2011 peak enjoyed by few, the early-adopter window is arguably still open.

===Won't loss of wallets and the finite amount of Bitcoins create excessive deflation, destroying Bitcoin? ===
Worries about Bitcoin being destroyed by deflation are not entirely unfounded. Unlike most currencies, which experience inflation as their founding institutions create more and more units, Bitcoin will likely experience gradual deflation with the passage of time. Bitcoin is unique in that only a small amount of units will ever be produced (twenty-one million to be exact), this number has been known since the project's inception, and the units are created at a predictable rate.

Also, Bitcoin users are faced with a danger that doesn't threaten users of any other currency: if a Bitcoin user loses his wallet, his money is gone forever, unless he finds it again. And not just to him; it's gone completely out of circulation, rendered utterly inaccessible to anyone. As people will lose their wallets, the total number of Bitcoins will slowly decrease.

Therefore, Bitcoin seems to be faced with a unique problem. Whereas most currencies inflate over time, Bitcoin will mostly likely do just the opposite. Time will see the irretrievable loss of an ever-increasing number of Bitcoins. An already small number will be permanently whittled down further and further. And as there become fewer and fewer Bitcoins, the laws of supply and demand suggest that their value will probably continually rise.

Thus Bitcoin is bound to once again stray into mysterious territory, because no one exactly knows what happens to a currency that grows continually more valuable. Many economists claim that a low level of inflation is a good thing for a currency, but nobody is quite sure about what might happens to one that continually deflates. Although deflation could hardly be called a rare phenomenon, steady, constant deflation is unheard of. There may be a lot of speculation, no one has any hard data to back up their claims.

That being said, there is a mechanism in place to combat the obvious consequences. Extreme deflation would render most currencies highly impractical: if a single Canadian dollar could suddenly buy the holder a car, how would one go about buying bread or candy? Even pennies would fetch more than a person could carry. Bitcoin, however, offers a simple and stylish solution: infinite divisibility. Bitcoins can be divided up and trade into as small of pieces as one wants, so no matter how valuable Bitcoins become, one can trade them in practical quantities.

In fact, infinite divisibility should allow Bitcoins to function in cases of extreme wallet loss. Even if, in the far future, so many people have lost their wallets that only a single Bitcoin, or a fraction of one, remains, Bitcoin should continue to function just fine. No one can claim to be sure what is going to happen, but deflation may prove to present a smaller threat than many expect.

For more information, see the [[Deflationary spiral]] page.

=== What if someone bought up all the existing Bitcoins? ===
Bitcoin markets are competitive -- meaning the price of a bitcoin will rise or fall depending on supply and demand at certain price levels. Only a fraction of bitcoins issued to date are found on the exchange markets for sale. So even though technically a buyer with lots of money could buy all the bitcoins offered for sale, unless those holding the rest of the bitcoins offer them for sale as well, even the wealthiest, most determined buyer can't get at them.

Additionally, new currency continues to be issued daily and will continue to do so for decades though over time the rate at which they are issued declines to insignificant levels. Those who are mining aren't obligated to sell their bitcoins so not all bitcoins will make it to the markets even.

This situation doesn't suggest, however, that the markets aren't vulnerable to price manipulation. It doesn't take significant amounts of money to move the market price up or down and thus Bitcoin remains a volatile asset.

===What if someone creates a new block chain, or a new digital currency that renders Bitcoin obsolete?===

That the block chain cannot be easily forked represents one of the central security mechanisms of Bitcoin. Given the choice between two block chains, a Bitcoin miner always chooses the longer one - that is to say, the one with the more complex hash. Thusly, it ensures that each user can only spend their bitcoins once, and that no user gets ripped off.

As a consequence of the block chain structure, there may at any time be many different sub-branches, and the possibility always exists of a transaction being over-written by the longest branch, if it has been recorded in a shorter one. The older a transaction is though, the lower its chances of being over-written, and the higher of becoming permanent. Although the block chain prevents one from spending more Bitcoins than one has, it means that transactions can be accidentally nullified.

A new block chain would leave the network vulnerable to [[double-spending|double-spend]] attacks. However, the creation of a viable new chain presents considerable difficulty, and the possibility does not present much of a risk.

Bitcoin will always choose the longer Block Chain and determines the relative length of two branches by the complexities of their hashes. Since the hash of each new block is made from that of the block preceding it, to create a block with a more complex hash, one must be prepared to do more computation than has been done by the entire Bitcoin network from the fork point up to the newest of the blocks one is trying to supersede. Needless to say, such an undertaking would require a very large amount of processing power and since Bitcoin is continually growing and expanding, it will likely only require more with the passage of time.

A much more distinct and real threat to the Bitcoin use is the development of other, superior virtual currencies, which could supplant Bitcoin and render it obsolete and valueless.

A great deal of careful thought and ingenuity has gone into the development of Bitcoin, but it is the first of its breed, a prototype, and vulnerable to more highly-evolved competitors. At present, any threatening rivals have yet to rear their heads; Bitcoin remains the first and foremost private virtual currency, but we can offer no guarantees that it will retain that position. It would certainly be in keeping with internet history for a similar system built from the same principles to supersede and cast Bitcoin into obsolescence, after time had revealed its major shortcomings. Friendster and Myspace suffered similar fates at the hand of Facebook, Napster was ousted by Limeware, Bearshare and torrent applications, and Skype has all but crushed the last few disciples of the Microsoft Messenger army.

This may sound rather foreboding, so bear in mind that the introduction of new and possibly better virtual currencies will not necessarily herald Bitcoin's demise. If Bitcoin establishes itself sufficiently firmly before the inception of the next generation of private, online currencies so as to gain widespread acceptance and general stability, future currencies may pose little threat even if they can claim superior design. This is known as the network effect.

=== Is Bitcoin open to value manipulation? ===

The current low market cap of Bitcoin means that any investor with deep enough pockets can significantly change/manipulate the rate. Is this a problem?

This is only a problem if you are investing in Bitcoin for short period of time. A manipulator can't change the fundamentals, and over a period of 5-10 years they will win over any short term manipulations.

==Sending and Receiving Payments==

=== Why do I have to wait 10 minutes before I can spend money I received? ===

10 minutes is the average time taken to find a block. It can be significantly more or less time than that depending on luck; 10 minutes is simply the average case.

You can see how long all other recent transactions have taken here: [http://bitcoinstats.org/ BitcoinStats.org].

[[Blocks]] (shown as "confirmations" in the GUI) are how the Bitcoin achieves consensus on who owns what. Once a block is found everyone agrees that you now own those coins, so you can spend them again. Until then it's possible that some network nodes believe otherwise, if somebody is attempting to defraud the system by reversing a transaction. The more confirmations a transaction has, the less risk there is of a reversal. Only 6 blocks or 1 hour is enough to make reversal computationally impractical. This is dramatically better than credit cards which can see chargebacks occur up to three months after the original transaction!

Ten minutes was specifically chosen by [[Satoshi]] as a tradeoff between first confirmation time and the amount of work wasted due to chain splits. After a block is mined, it takes time for other miners to find out about it, and until then they are actually competing against the new block instead of adding to it. If someone mines another new block based on the old block chain, the network can only accept one of the two, and all the work that went into the other block gets wasted. For example, if it takes miners 1 minute on average to learn about new blocks, and new blocks come every 10 minutes, then the overall network is wasting about 10% of its work. Lengthening the time between blocks reduces this waste.

As a thought experiment, what if the Bitcoin network grew to include Mars? From the farthest points in their orbits, it takes about 20 minutes for a signal to travel from Earth to Mars. With only 10 minutes between new blocks, miners on Mars would always be 2 blocks behind the miners on Earth. It would be almost impossible for them to contribute to the block chain. If we wanted collaborate with those kinds of delays, we would need at least a few hours between new blocks.

[[File:TransactionConfirmationTimesExample.PNG]]

=== Do you have to wait until my transactions are confirmed in order to buy or sell things with Bitcoin? ===

YES, you do, IF the transaction is non-recourse. The Bitcoin reference software does not display transactions as confirmed until six blocks have passed (confirmations). As transactions are burred in the chain they become increasingly non-reversible but are very reversible before the first confirmation. Two to six confirmations are recommended for non-recourse situations depending on the value of the transactions involved.

When people ask this question they are usually thinking about applications like supermarkets. This generally is a recourse situation: if somebody tries to double-spend on a face-to-face transaction it might work a few times, but probabalistically speaking eventually one of the double-spends will get noticed, and the penalty for shoplifting charges in most localities is calibrated to be several times worse than the proceeds of a single shoplifting event.

Double-spends might be a concern for something like a snack machine in a low-traffic area with no nearby security cameras. Such a machine shouldn't honor 0-confirmation payments, and should instead use some other mechanism of clearing Bitcoin or validating transactions against reversal, see the wiki article [[Myths#Point_of_sale_with_bitcoins_isn.27t_possible_because_of_the_10_minute_wait_for_confirmation|here]] for alternatives.

Applications that require immediate payment processing, like supermarkets or snack machines, need to manage the risks. Here is one way to reverse an unconfirmed payment:

A [[Double-spending#Finney_attack|Finney attack]], in which an attacker mines a block containing a movement of some coins back to themselves. Once they find a block solution, they quickly go to a merchant and make a purchase, then broadcast the block, thus taking back the coins. This attack is a risk primarily for goods that are dispatched immediately, like song downloads or currency trades. Because the attacker can't choose the time of the attack, it isn't a risk for merchants such as supermarkets where you can't choose exactly when to pay (due to queues, etc). The attack can fail if somebody else finds a block containing the purchasing transaction before you release your own block, therefore, merchants can reduce but not eliminate the risk by making purchasers wait some length of time that's less than a confirm.

Because pulling off this attack is not trivial, merchants who need to sell things automatically and instantly are most likely to just price the cost of reversal fraud in, or use insurance.

=== I was sent some bitcoins and they haven't arrived yet! Where are they? ===

Don't panic! There are a number of reasons why your bitcoins might not show up yet, and a number of ways to diagnose them.

The latest version of the Bitcoin-Qt client tells you how far it has yet to go in downloading the blockchain. Hover over the icon in the bottom right corner of the client to learn your client's status.

If it has not caught up then it's possible that your transaction hasn't been included in a block yet.

You can check pending transactions in the network by going [http://blockchain.info here] and then searching for your address. If the transaction is listed here then it's a matter of waiting until it gets included in a block before it will show in your client.

If the transaction is based on a coin that was in a recent transaction then it could be considered a low priority transaction. Transfers can take longer if the transaction fee paid was not high enough. If there is no fee at all the transfer can get a very low priority and take hours or even days to be included in a block.

=== Why does my Bitcoin address keep changing? ===

Whenever the address listed in "Your address" receives a transaction, Bitcoin replaces it with a new address. This is meant to encourage you to use a new address for every transaction, which enhances [[anonymity]]. All of your old addresses are still usable: you can see them in ''Settings -> Your Receiving Addresses''.

===How much will the transaction fee be?===

Some transactions might require a [[transaction fee]] for them to get confirmed in a timely manner. The transaction fee is processed by and received by the bitcoin miner. The most recent version of the Bitcoin client will estimate an appropriate fee when a fee might be required.

The fee is added to the payment amount. For example, if you are sending a 1.234 BTC payment and the client requires a 0.0005 BTC fee, then 1.2345 BTC will be subtracted from the wallet balance for the entire transaction and the address for where the payment was sent will receive a payment of 1.234 BTC.

A fee might be imposed because your transaction looks like a denial of service attack to the Bitcoin system. For example, it might be burdensome to transmit or it might recycle Bitcoins you recently received. The wallet software attempts to avoid generating burdensome transactions, but it isn't always able to do so: The funds in your wallet might be new or composed of many tiny payments.

Because the fee is related to the amount of data that makes up the transaction and not to the amount of Bitcoins being sent, the fee may seem extremely low (0.0005 BTC for a 1,000 BTC transfer) or unfairly high (0.004 BTC for a 0.02 BTC payment, or about 20%). If you are receiving tiny amounts (''e.g.'' as small payments from a mining pool) then fees when sending will be higher than if your activity follows the pattern of conventional consumer or business transactions.

As of Bitcoin 0.5.3 the required fee will not be higher than 0.05 BTC. For most users there is usually no required fee at all. If a fee is required it will most commonly be 0.0005 BTC.

=== What happens when someone sends me a bitcoin but my computer is powered off? ===

Bitcoins are not actually "sent" to your wallet; the software only uses that term so that we can use the currency without having to learn new concepts. Your wallet is only needed when you wish to spend coins that you've received.

If you are sent coins when your wallet client program is not running, and you later launch the wallet client program, the coins will eventually appear as if they were just received in the wallet. That is to say, when the client program is started it must download blocks and catch up with any transactions it did not already know about.

=== How long does "synchronizing" take when the Bitcoin client is first installed? What's it doing? ===

The popular Bitcoin client software from bitcoin.org implements a "full" Bitcoin node: It can carry out all the duties of the Bitcoin P2P system, it isn't simply a "client". One of the principles behind the operation of full Bitcoin nodes is that they don't assume that the other participants have followed the rules of the Bitcoin system. During synchronization, the software is processing historical Bitcoin transactions and making sure for itself that all of the rules of the system have been correctly followed.

In normal operation, after synchronizing, the software should use a hardly noticeable amount of your computer's resources.

When the wallet client program is first installed, its initial validation requires a lot of work from your computer's hard disk, so the amount of time to synchronize depends on your disk speed and, to a lesser extent, your CPU speed. It can take anywhere from a few hours to a day or so. On a slow computer it could take more than 40 hours of continuous synchronization, so check your computer's power-saving settings to ensure that it does not turn its hard disk off when unattended for a few hours. You can use the Bitcoin software during synchronization, but you may not see recent payments to you until the client program has caught up to the point where those transactions happened.

If you feel that this process takes too long, you can download a pre-synchronized blockchain from [http://eu2.bitcoincharts.com/blockchain/ http://eu2.bitcoincharts.com/blockchain/]. Alternatively, you can try an alternative "lite" client such as Multibit or a super-light client like electrum, though these clients have somewhat weaker security, are less mature, and don't contribute to the health of the P2P network.

==Networking==
=== Do I need to configure my firewall to run Bitcoin? ===

Bitcoin will connect to other nodes, usually on TCP port 8333. You will need to allow outgoing TCP connections to port 8333 if you want to allow your Bitcoin client to connect to many nodes. [[Testnet]] uses TCP port 18333 instead of 8333.

If you want to restrict your firewall rules to a few IPs, you can find stable nodes in the [[Fallback Nodes|fallback nodes list]].

=== How does the peer finding mechanism work? ===

Bitcoin finds peers primarily by forwarding peer announcements within its own network and each node saves a database of peers that it's aware of, for future use. In order to bootstrap this process Bitcoin needs a list of initial peers, these can be provided manually but normally it obtains them by querying a set of DNS domain names which have automatically updated lists, if that doesn't work it falls back to a built-in list which is updated from time to time in new versions of the software. There is also an IRC based mechanism but it is disabled by default.

==Mining==
===What is mining?===
[[Mining]] is the process of spending computation power to secure Bitcoin transactions against reversal and introducing new Bitcoins to the system.

Technically speaking, mining is the calculation of a [[hash]] of the a block header, which includes among other things a reference to the previous block, a hash of a set of transactions and a [[nonce]]. If the hash value is found to be less than the current [[target]] (which is inversely proportional to the [[difficulty]]), a new block is formed and the miner gets the newly generated Bitcoins (25 per block at current levels). If the hash is not less than the current target, a new nonce is tried, and a new hash is calculated. This is done millions of times per second by each miner.

===Is mining used for some useful computation?===
The computations done when mining are internal to Bitcoin and not related to any other distributed computing projects. They serve the purpose of securing the Bitcoin network, which is useful.

===Is it not a waste of energy?===
Spending energy on creating and securing a free monetary system is hardly a waste. Also, services necessary for the operation of currently widespread monetary systems, such as banks and credit card companies, also spend energy, arguably more than Bitcoin would.

===Why don't we use calculations that are also useful for some other purpose?===
To provide security for the Bitcoin network, the calculations involved need to have some [http://bitcoin.stackexchange.com/questions/5617/why-are-bitcoin-calculation-useless/5618#5618 very specific features]. These features are incompatible with leveraging the computation for other purposes.

===How can we stop miners from creating zero transaction blocks?===
The incentive for miners to include transactions is in the fees that come along with them. If we were to implement some minimum number of transactions per block it would be trivial for a miner to create and include transactions merely to surpass that threshold. As the network matures, the block reward drops, and miners become more dependent on transactions fees to pay their costs, the problem of zero transaction blocks should diminish over time.

===How does the proof-of-work system help secure Bitcoin?===
To give a general idea of the mining process, imagine this setup:

payload = <some data related to things happening on the Bitcoin network>
nonce = 1
hash = [http://en.wikipedia.org/wiki/SHA2 SHA2]( [http://en.wikipedia.org/wiki/SHA2 SHA2]( payload + nonce ) )

The work performed by a miner consists of repeatedly increasing "nonce" until
the hash function yields a value, that has the rare property of being below a certain
target threshold. (In other words: The hash "starts with a certain number of zeroes",
if you display it in the fixed-length representation, that is typically used.)

As can be seen, the mining process doesn't compute anything special. It merely
tries to find a number (also referred to as nonce) which - in combination with the payload -
results in a hash with special properties.

The advantage of using such a mechanism consists of the fact, that it is very easy to check a result: Given the payload and a specific nonce, only a single call of the hashing function is needed to verify that the hash has the required properties. Since there is no known way to find these hashes other than brute force, this can be used as a "proof of work" that someone invested a lot of computing power to find the correct nonce for this payload.

This feature is then used in the Bitcoin network to secure various aspects. An attacker
that wants to introduce malicious payload data into the network, will need to do the
required proof of work before it will be accepted. And as long as honest miners have more
computing power, they can always outpace an attacker.

Also see [http://en.wikipedia.org/wiki/Hashcash Hashcash] and [http://en.wikipedia.org/wiki/Proof-of-work_system Proof-of-work system] and [http://en.wikipedia.org/wiki/SHA2 SHA2] and on Wikipedia.

===Why was the "Generate coin" option of the client software removed?===

In the early days of Bitcoin, it was easy for anyone to find new blocks using standard CPUs. As more and more people started mining, the [[difficulty]] of finding new blocks has greatly increased to the point where the average time for a CPU to find a single block can be many years. The only cost-effective method of [[Mining|mining]] is using a high-end graphics card with special software (see also [[Why a GPU mines faster than a CPU]]) and/or joining a [[Bitcoin Pool|mining pool]]. Since solo CPU mining is essentially useless, it was removed from the GUI of the Bitcoin software.

==Security==

===Could miners collude to give themselves money or to fundamentally change the nature of Bitcoin?===

There are two questions in here. Let's look at them separately.

;Could miners gang up and give themselves money?

Mining itself is the process of creating new blocks in the block chain. Each block contains a list of all the transactions that have taken place across the entire Bitcoin network since the last block was created, as well as a hash of the previous block. New blocks are 'mined', or rather, generated, by Bitcoin clients correctly guessing sequences of characters in codes called 'hashes,' which are created using information from previous blocks. Bitcoin users may download specialized 'mining' software, which allows them to dedicate some amount of their processing power – however large or small – to guessing at strings within the hash of the previous block. Whoever makes the right guess first, thus creating a new block, receives a reward in Bitcoins.

The block chain is one of the two structures that makes Bitcoin secure, the other being the public-key encryption system on which Bitcoin trade is based. The block chain assures that not only is every single transaction that ever takes place recorded, but that every single transaction is recorded on the computer of anyone who chooses to store the relevant information. Many, many users have complete records of every transaction in Bitcoins history readily available to them at any point, and anyone who wants in the information can obtain it with ease. These things make Bitcoin very hard to fool.

The Bitcoin network takes considerable processing power to run, and since those with the most processing power can make the most guesses, those who put the most power toward to sustaining the network earn the most currency. Each correct guess yields, at present, twenty-five Bitcoins, and as Bitcoins are presently worth something (although the value still fluctuates) every miner who earns any number of Bitcoins makes money. Some miners pull in Bitcoins on their own; and some also join or form pools wherein all who contribute earn a share of the profits.

Therefore, first answer is a vehement “yes” – not only can miners collude to get more money, Bitcoin is designed to encourage them to do so. Bitcoin pools are communal affairs, and there is nothing dishonest or underhanded about them.

Of course, the real question is:

;Can they do so in ways not sanctioned by Bitcoin developers? Is there any way to rip off the network and make loads of money dishonestly?

Bitcoin isn't infallible. It can be cheated, but doing so is extremely difficult. Bitcoin was designed to evade some of the central problems with modern currencies – namely, that their trustworthiness hinges upon that of people who might not have users' best interests in mind. Every currency in the world (other than Bitcoin) is controlled by large institutions who keep track of what's done with it, and who can manipulate its value. And every other currency has value because people trust the institutions that control them.

Bitcoin doesn't ask that its users trust any institution. Its security is based on the cryptography that is an integral part of its structure, and that is readily available for any and all to see. Instead of one entity keeping track of transactions, the entire network does, so Bitcoins are astoundingly difficult to steal, or double-spend. Bitcoins are created in a regular and predictable fashion, and by many different users, so no one can decide to make a whole lot more and lessen their value. In short, Bitcoin is designed to be inflation-proof, double-spend-proof and completely distributed.

Nonetheless, there are a few ways that one can acquire Bitcoins dishonestly. Firstly, one can steal private keys. Key theft isn't something that Bitcoin security has been designed to prevent: it's up to users to keep theirs safe. But the cryptography is designed so that it is completely impossible to deduce someone's private key from their public one. As long as you keep your private key to yourself, you don't have much to worry about. Furthermore, one could theoretically create a new block chain, but due to the way in which the block chain is constructed, this would be extremely difficult and require massive amounts of processing power. A full explanation of the difficulties involved can be found in the [[block chain]] article.

Bitcoin can be ripped off – but doing so would be extremely hard and require considerable expertise and a staggering amount of processing power. And it's only going to get harder with time. Bitcoin isn't impenetrable, but it's close enough to put any real worries in the peripherals.

;Could miners fundamentally change the nature of Bitcoin?

Once again, almost certainly not.

Bitcoin is a distributed network, so any changes implemented to the system must be accepted by all users. Someone trying to change the way Bitcoins are generated would have to convince every user to download and use their software – so the only changes that would go through are those that would be equally benefit all users.

And thus, it is more or less impossible for anyone to change the function of Bitcoin to their advantage. If users don't like the changes, they won't adopt them, whereas if users do like them, then these will help everyone equally. Of course, one can conceive of a situation where someone manages to get a change pushed through that provides them with an advantage that no one notices, but given that Bitcoin is structurally relatively simple, it is unlikely that any major changes will go through without someone noticing first.

The fact that such changes are so difficult to make testifies to the fully distributed nature of Bitcoin. Any centrally controlled currency can be modified by its central agency without the consent of its adherents. Bitcoin has no central authority, so it changes only at the behest of the whole community. Bitcoins development represents a kind of collective evolution; the first of its kind among currencies.

==Help==
===I'd like to learn more. Where can I get help?===

* Read the [[Introduction|introduction to bitcoin]]
* See the videos, podcasts, and blog posts from the [[Press]]
* Read and post on the [[:Bitcoin:Community_portal#Bitcoin_Community_Forums|forums]]
* Chat on one of the [[:Bitcoin:Community_portal#IRC_Chat|Bitcoin IRC]] channels
* Listen to [http://omegataupodcast.net/2011/03/59-bitcoin-a-digital-decentralized-currency/ this podcast], which goes into the details of how bitcoin works
* Ask questions on the [http://bitcoin.stackexchange.com Bitcoin Stack Exchange]

==See Also==

* [[Man page]]
* [[Introduction]]

[[de:FAQ]]
[[zh-cn:FAQ]]
[[fr:FAQ]]
[[ru:FAQ]]

[[Category:Technical]]
[[Category:Vocabulary]]

Help:FAQ

2013-06-27T10:43:55Z

Oconnor663: Explaining the 10 minute time between blocks -- please make corrections.

Here you will find answers to the most commonly asked questions.

== General ==
=== What are bitcoins? ===
Bitcoins are the unit of currency of the Bitcoin system. A commonly used shorthand for this is “BTC” to refer to a price or amount (eg: “100 BTC”).
There are such things as [[physical bitcoins]], but ultimately, a bitcoin is just a number associated with a [[Address|Bitcoin Address]]. A physical bitcoin is simply an object, such as a coin, with the number carefully embedded inside. See also an [[Introduction|easy intro]] to bitcoin.

=== How can I get bitcoins? ===

There are a variety of ways to acquire bitcoins:

* Accept bitcoins as payment for goods or services.
* The most common way to buy bitcoins are the [[Buying bitcoins|Bitcoin Exchanges]]
* There are several services where you can [[Buying_Bitcoins_(the_noob_version)|trade them]] for traditional currency.
* Find someone to trade cash for bitcoins in-person through a [https://en.bitcoin.it/wiki/Category:Directories local directory].
* Participate in a [[Pooled mining|mining pool]].
* If you have a lot of mining hardware, you can solo mine and attempt to create a new [[block]] (currently yields 25 bitcoins plus transaction fees).
* Visit sites that provide [[Trade#Free_Samples_and_Offers|free samples and offers]].

===Does Bitcoin guarantee an influx of free money?===

Since Bitcoin is a new technology, what it is and how it works may be initially unclear. Bitcoin is sometimes presented as being one of three things:
<ol style="list-style-type: upper-alpha;">
<li>Some sort of online 'get-rich-quick' scam.</li>
<li>A loophole in the market economy, the installation of which guarantees a steady influx of cash.</li>
<li>A sure investment that will almost certainly yield a profit.</li>
</ol>
In fact, none of the above are true. Let's look at them independently.

;Is Bitcoin a 'get-rich-quick' scheme?
:If you've spent much time on the Internet, you've probably seen ads for many 'get-rich-quick' schemes. These ads usually promise huge profits for a small amounts of easy work. Such schemes are usually pyramid/matrix-style schemes that make money from their own employees and offer nothing of any real value. Most convince one to buy packages that will make them earn hundreds a day, which in fact have the buyer distribute more such ads, and make minute profits.

:Bitcoin is in no way similar to these schemes. Bitcoin doesn't promise windfall profits. There is no way for the developers to make money from your involvement or to take money from you. That bitcoins are nearly impossible to acquire without the owner's consent represents one of its greatest strengths. Bitcoin is an experimental, virtual currency that may succeed or may fail. None of its developers expect to get rich off of it.

:A more detailed answer to this question can be found [http://bitcointalk.org/?topic=7815.0 here].

;Will I make money by installing the client?
:Most people who use Bitcoin don't earn anything by doing so, and the default client has no built-in way to earn Bitcoins. A small minority of people with dedicated, high-performance hardware do earn some Bitcoins by "''mining''" (generating new bitcoins, see [[#What is mining?|What is mining?]]) with special software, but joining Bitcoin shouldn't be construed as being the road to riches. Most Bitcoin users get involved because they find the project conceptually interesting and don't earn anything by doing so. This is also why you won't find much speculation about the political or economic repercussions of Bitcoin anywhere on this site: Bitcoin developers owe their dedication to the project's intellectual yieldings more than to those of a monetary nature. Bitcoin is still taking its first baby steps; it may go on to do great things but right now it only has something to offer those chasing conceptually interesting projects or bleeding edge technology.

;As an investment, is Bitcoin a sure thing?
:Bitcoin is a new and interesting electronic currency, the value of which is not backed by any single government or organization. Like other currencies, it is worth something partly because people are willing to trade it for goods and services. Its exchange rate fluctuates continuously, and sometimes wildly. It lacks wide acceptance and is vulnerable to manipulation by parties with modest funding. Security incidents such as website and account compromise may trigger major sell-offs. Other fluctuations can build into positive feedback loops and cause much larger exchange rate fluctuations. Anyone who puts money into Bitcoin should understand the risk they are taking and consider it a high-risk currency. Later, as Bitcoin becomes better known and more widely accepted, it may stabilize, but for the time being it is unpredictable. Any investment in Bitcoin should be done carefully and with a clear plan to manage the risk.

=== Can I buy bitcoins with Paypal? ===

It is possible to buy [[physical bitcoins]] with PayPal but it is otherwise difficult and/or expensive to do so for non-physical bitcoins, because of significant risk to the seller.

While it is possible to find an individual who wishes to sell Bitcoin to you via Paypal, (perhaps via [http://www.bitcoin-otc.com/ #bitcoin-otc] ) most exchanges do not allow funding through PayPal. This is due to repeated cases where someone pays for bitcoins with Paypal, receives their bitcoins, and then fraudulently complains to Paypal that they never received their purchase. PayPal often sides with the fraudulent buyer in this case, which means any seller needs to cover that risk with higher fees or refuse to accept PayPal altogether.

Buying Bitcoins from individuals this way is still possible, but requires the seller to have some trust that the buyer will not file a claim with PayPal to reverse the payment.

=== Where can I find a forum to discuss Bitcoin? ===

Please visit the [[Bitcoin:Community_portal#Bitcoin_Community_Forums_on_various_platforms|Community Portal]] for links to Bitcoin-related forums.

=== How are new bitcoins created? ===

[[File:total_bitcoins_over_time_graph.png|thumb|Number of bitcoins over time, assuming a perfect 10-minute interval.]]
New bitcoins are generated by the network through the process of "[[#What is mining?|''mining'']]". In a process that is similar to a continuous raffle draw, mining nodes on the network are awarded bitcoins each time they find the solution to a certain mathematical problem (and thereby create a new [[block]]). Creating a block is a [[proof of work]] with a difficulty that varies with the overall strength of the network. The reward for solving a block is [[Controlled Currency Supply|automatically adjusted]] so that roughly every four years of operation of the Bitcoin network, half the amount of bitcoins created in the prior 4 years are created. {{formatnum:10500000}} bitcoins were created in the first 4 (approx.) years from January 2009 to November 2012. Every four years thereafter this amount halves, so it will be {{formatnum:5250000}} over years 4-8, {{formatnum:2625000}} over years 8-12, and so on. Thus the total number of bitcoins in existence will never exceed {{formatnum:21000000}}. See [[Controlled Currency Supply]].

Blocks are [[Mining|mined]] every 10 minutes, on average and for the first four years ({{formatnum:210000}} blocks) each block included 50 new bitcoins. As the amount of processing power directed at mining changes, the difficulty of creating new bitcoins changes. This difficulty factor is calculated every 2016 blocks and is based upon the time taken to generate the previous 2016 blocks. See [[Mining]].

=== What's the current total number of bitcoins in existence? ===

[http://blockexplorer.com/q/totalbc Current count]. Also see [https://blockchain.info/charts/total-bitcoins Total bitcoins in circulation chart]

The number of blocks times the coin value of a block is the number of coins in existence. The coin value of a block is 50 BTC for each of the first {{formatnum:210000}} blocks, 25 BTC for the next {{formatnum:210000}} blocks, then 12.5 BTC, 6.25 BTC and so on.

=== How divisible are bitcoins? ===

A bitcoin can be divided down to 8 decimal places. Therefore, 0.00000001 BTC is the smallest amount that can be handled in a transaction. If necessary, the protocol and related software can be modified to handle even smaller amounts.

=== What do I call the various denominations of bitcoin? ===

There is a lot of discussion about the naming of these fractions of bitcoins. The leading candidates are:

* 1 BTC = 1 bitcoin
* 0.01 BTC = 1 cBTC = 1 centibitcoin (also referred to as bitcent)
* 0.001 BTC = 1 mBTC = 1 millibitcoin (also referred to as mbit (pronounced em-bit) or millibit or even bitmill)
* 0.000 001 BTC = 1 μBTC = 1 microbitcoin (also referred to as ubit (pronounced yu-bit) or microbit)

The above follows the accepted international SI prefixes for hundredths, thousandths, and millionths. There are many arguments against the special case of 0.01 BTC since it is unlikely to represent anything meaningful as the Bitcoin economy grows (it certainly won't be the equivalent of 0.01 USD, GBP or EUR). Equally, the inclusion of existing national currency denominations such as "cent", "nickel", "dime", "pence", "pound", "kopek" and so on are to be discouraged; this is a worldwide currency.

One exception is the "satoshi" which is smallest denomination currently possible

* 0.000 000 01 BTC = 1 satoshi (pronounced sa-toh-shee)
which is so named in honour of Satoshi Nakamoto, the pseudonym of the inventor of Bitcoin.

For an overview of all defined units of Bitcoin (including less common and niche units), see [[Units]].

Further discussion on this topic can be found on the forums here:

* [https://bitcointalk.org/index.php?topic=14438.msg195287#msg195287 We need names]
* [https://bitcointalk.org/index.php?topic=8282.0 What to call 0.001 BTC]

=== How does the halving work when the number gets really small? ===

Eventually the reward will go from 0.00000001 BTC to zero and no more bitcoins will be created.

The block reward calculation is done as a right bitwise shift of a 64-bit signed integer, which means it is divided by two and rounded down. The integer is equal to the value in BTC * 100,000,000 since internally in the reference client software, all Bitcoin balances and values are stored as unsigned integers.

With an initial block reward of 50 BTC, it will take many 4-year periods for the block reward to reach zero.

=== How long will it take to generate all the coins? ===

The last block that will generate coins will be block #6,929,999 which should be generated at or near the year 2140. The total number of coins in circulation will then remain static at 20,999,999.9769 BTC.

Even if the allowed precision is expanded from the current 8 decimals, the total BTC in circulation will always be slightly below 21 million (assuming everything else stays the same). For example, with 16 decimals of precision, the end total would be 20,999,999.999999999496 BTC.

=== If no more coins are going to be generated, will more blocks be created? ===

Absolutely! Even before the creation of coins ends, the use of [[transaction fee|transaction fees]] will likely make creating new blocks more valuable from the fees than the new coins being created. When coin generation ends, these fees will sustain the ability to use bitcoins and the Bitcoin network. There is no practical limit on the number of blocks that will be mined in the future.

=== But if no more coins are generated, what happens when Bitcoins are lost? Won't that be a problem? ===

Because of the law of supply and demand, when fewer bitcoins are available the ones that are left will be in higher demand, and therefore will have a higher value. So, as Bitcoins are lost, the remaining bitcoins will eventually increase in value to compensate. As the value of a bitcoin increases, the number of bitcoins required to purchase an item '''de'''creases. This is a [[Deflationary spiral|deflationary economic model]]. As the average transaction size reduces, transactions will probably be denominated in sub-units of a bitcoin such as millibitcoins ("Millies") or microbitcoins ("Mikes").

The Bitcoin protocol uses a base unit of one hundred-millionth of a Bitcoin ("a Satoshi"), but unused bits are available in the protocol fields that could be used to denote even smaller subdivisions.

=== If every transaction is broadcast via the network, does Bitcoin scale? ===
The Bitcoin protocol allows lightweight clients that can use Bitcoin without downloading the entire transaction history. As traffic grows and this becomes more critical, implementations of the concept will be developed. Full network nodes will at some point become a more specialized service.

With some modifications to the software, full Bitcoin nodes could easily keep up with both VISA and MasterCard combined, using only fairly modest hardware (a single high end server by todays standards). It is worth noting that the MasterCard network is structured somewhat like Bitcoin itself - as a peer to peer broadcast network.

Learn more about [[Scalability]].

==Economy==
=== Where does the value of Bitcoin stem from? What backs up Bitcoin? ===
Bitcoins have value because they are useful and because they are [[Controlled Currency Supply|scarce]]. As they are accepted by more merchants, their value will [http://en.wikipedia.org/wiki/Sticky_%28economics%29 stabilize]. See the [[Trade|list of Bitcoin-accepting sites]].

When we say that a currency is backed up by gold, we mean that there's a promise in place that you can exchange the currency for gold. Bitcoins, like dollars and euros, are not backed up by anything except the variety of merchants that accept them.

It's a common misconception that Bitcoins gain their value from the cost of electricity required to generate them. Cost doesn't equal value – hiring 1,000 men to shovel a big hole in the ground may be costly, but not valuable. Also, even though scarcity is a critical requirement for a useful currency, it alone doesn't make anything valuable. For example, your fingerprints are scarce, but that doesn't mean they have any exchange value.

Alternatively it needs to be added that while the law of supply and demand applies it does not guarantee value of Bitcoins in the future. If confidence in Bitcoins is lost then it will not matter that the supply can no longer be increased, the demand will fall off with all holders trying to get rid of their coins. An example of this can be seen in cases of state currencies, in cases when the state in question dissolves and so no new supply of the currency is available (the central authority managing the supply is gone), however the demand for the currency falls sharply because confidence in its purchasing power disappears. Of-course Bitcoins do not have such central authority managing the supply of the coins, but it does not prevent confidence from eroding due to other situations that are not necessarily predictable.

=== Is Bitcoin a bubble? ===
Yes, in the same way as the euro and dollar are. They only have value in exchange and have no inherent value. If everyone suddenly stopped accepting your dollars, euros or bitcoins, the "bubble" would burst and their value would drop to zero. But that is unlikely to happen: even in Somalia, where the government collapsed 20 years ago, [http://en.wikipedia.org/wiki/Somali_shilling Somali shillings] are still accepted as payment.

=== Is Bitcoin a Ponzi scheme? ===
In a Ponzi Scheme, the founders persuade investors that they’ll profit. Bitcoin does not make such a guarantee. There is no central entity, just individuals building an economy.

A ponzi scheme is a zero sum game. Early adopters can only profit at the expense of late adopters. Bitcoin has possible win-win outcomes. Early adopters profit from the rise in value. Late adopters, and indeed, society as a whole, benefit from the usefulness of a stable, fast, inexpensive, and widely accepted p2p currency.

The fact that early adopters benefit more doesn't alone make anything a Ponzi scheme. All good investments in successful companies have this quality.

=== Doesn't Bitcoin unfairly benefit early adopters? ===
Early adopters have a large number of bitcoins now because they took a risk and invested resources in an unproven technology. By so doing, they have helped Bitcoin become what it is now and what it will be in the future (hopefully, a ubiquitous decentralized digital currency). It is only fair they will reap the benefits of their successful investment.

In any case, any bitcoin generated will probably change hands dozens of time as a medium of exchange, so the profit made from the initial distribution will be insignificant compared to the total commerce enabled by Bitcoin.

Since the pricing of Bitcoins has fallen greatly from its June 2011 peak, prices today are much more similar to those enjoyed by many early adopters. Those who are buying Bitcoins today likely believe that Bitcoin will grow significantly in the future. Setting aside the brief opportunity to have sold Bitcoins at the June 2011 peak enjoyed by few, the early-adopter window is arguably still open.

===Won't loss of wallets and the finite amount of Bitcoins create excessive deflation, destroying Bitcoin? ===
Worries about Bitcoin being destroyed by deflation are not entirely unfounded. Unlike most currencies, which experience inflation as their founding institutions create more and more units, Bitcoin will likely experience gradual deflation with the passage of time. Bitcoin is unique in that only a small amount of units will ever be produced (twenty-one million to be exact), this number has been known since the project's inception, and the units are created at a predictable rate.

Also, Bitcoin users are faced with a danger that doesn't threaten users of any other currency: if a Bitcoin user loses his wallet, his money is gone forever, unless he finds it again. And not just to him; it's gone completely out of circulation, rendered utterly inaccessible to anyone. As people will lose their wallets, the total number of Bitcoins will slowly decrease.

Therefore, Bitcoin seems to be faced with a unique problem. Whereas most currencies inflate over time, Bitcoin will mostly likely do just the opposite. Time will see the irretrievable loss of an ever-increasing number of Bitcoins. An already small number will be permanently whittled down further and further. And as there become fewer and fewer Bitcoins, the laws of supply and demand suggest that their value will probably continually rise.

Thus Bitcoin is bound to once again stray into mysterious territory, because no one exactly knows what happens to a currency that grows continually more valuable. Many economists claim that a low level of inflation is a good thing for a currency, but nobody is quite sure about what might happens to one that continually deflates. Although deflation could hardly be called a rare phenomenon, steady, constant deflation is unheard of. There may be a lot of speculation, no one has any hard data to back up their claims.

That being said, there is a mechanism in place to combat the obvious consequences. Extreme deflation would render most currencies highly impractical: if a single Canadian dollar could suddenly buy the holder a car, how would one go about buying bread or candy? Even pennies would fetch more than a person could carry. Bitcoin, however, offers a simple and stylish solution: infinite divisibility. Bitcoins can be divided up and trade into as small of pieces as one wants, so no matter how valuable Bitcoins become, one can trade them in practical quantities.

In fact, infinite divisibility should allow Bitcoins to function in cases of extreme wallet loss. Even if, in the far future, so many people have lost their wallets that only a single Bitcoin, or a fraction of one, remains, Bitcoin should continue to function just fine. No one can claim to be sure what is going to happen, but deflation may prove to present a smaller threat than many expect.

For more information, see the [[Deflationary spiral]] page.

=== What if someone bought up all the existing Bitcoins? ===
Bitcoin markets are competitive -- meaning the price of a bitcoin will rise or fall depending on supply and demand at certain price levels. Only a fraction of bitcoins issued to date are found on the exchange markets for sale. So even though technically a buyer with lots of money could buy all the bitcoins offered for sale, unless those holding the rest of the bitcoins offer them for sale as well, even the wealthiest, most determined buyer can't get at them.

Additionally, new currency continues to be issued daily and will continue to do so for decades though over time the rate at which they are issued declines to insignificant levels. Those who are mining aren't obligated to sell their bitcoins so not all bitcoins will make it to the markets even.

This situation doesn't suggest, however, that the markets aren't vulnerable to price manipulation. It doesn't take significant amounts of money to move the market price up or down and thus Bitcoin remains a volatile asset.

===What if someone creates a new block chain, or a new digital currency that renders Bitcoin obsolete?===

That the block chain cannot be easily forked represents one of the central security mechanisms of Bitcoin. Given the choice between two block chains, a Bitcoin miner always chooses the longer one - that is to say, the one with the more complex hash. Thusly, it ensures that each user can only spend their bitcoins once, and that no user gets ripped off.

As a consequence of the block chain structure, there may at any time be many different sub-branches, and the possibility always exists of a transaction being over-written by the longest branch, if it has been recorded in a shorter one. The older a transaction is though, the lower its chances of being over-written, and the higher of becoming permanent. Although the block chain prevents one from spending more Bitcoins than one has, it means that transactions can be accidentally nullified.

A new block chain would leave the network vulnerable to [[double-spending|double-spend]] attacks. However, the creation of a viable new chain presents considerable difficulty, and the possibility does not present much of a risk.

Bitcoin will always choose the longer Block Chain and determines the relative length of two branches by the complexities of their hashes. Since the hash of each new block is made from that of the block preceding it, to create a block with a more complex hash, one must be prepared to do more computation than has been done by the entire Bitcoin network from the fork point up to the newest of the blocks one is trying to supersede. Needless to say, such an undertaking would require a very large amount of processing power and since Bitcoin is continually growing and expanding, it will likely only require more with the passage of time.

A much more distinct and real threat to the Bitcoin use is the development of other, superior virtual currencies, which could supplant Bitcoin and render it obsolete and valueless.

A great deal of careful thought and ingenuity has gone into the development of Bitcoin, but it is the first of its breed, a prototype, and vulnerable to more highly-evolved competitors. At present, any threatening rivals have yet to rear their heads; Bitcoin remains the first and foremost private virtual currency, but we can offer no guarantees that it will retain that position. It would certainly be in keeping with internet history for a similar system built from the same principles to supersede and cast Bitcoin into obsolescence, after time had revealed its major shortcomings. Friendster and Myspace suffered similar fates at the hand of Facebook, Napster was ousted by Limeware, Bearshare and torrent applications, and Skype has all but crushed the last few disciples of the Microsoft Messenger army.

This may sound rather foreboding, so bear in mind that the introduction of new and possibly better virtual currencies will not necessarily herald Bitcoin's demise. If Bitcoin establishes itself sufficiently firmly before the inception of the next generation of private, online currencies so as to gain widespread acceptance and general stability, future currencies may pose little threat even if they can claim superior design. This is known as the network effect.

=== Is Bitcoin open to value manipulation? ===

The current low market cap of Bitcoin means that any investor with deep enough pockets can significantly change/manipulate the rate. Is this a problem?

This is only a problem if you are investing in Bitcoin for short period of time. A manipulator can't change the fundamentals, and over a period of 5-10 years they will win over any short term manipulations.

==Sending and Receiving Payments==

=== Why do I have to wait 10 minutes before I can spend money I received? ===

10 minutes is the average time taken to find a block. It can be significantly more or less time than that depending on luck; 10 minutes is simply the average case.

You can see how long all other recent transactions have taken here: [http://bitcoinstats.org/ BitcoinStats.org].

[[Blocks]] (shown as "confirmations" in the GUI) are how the Bitcoin achieves consensus on who owns what. Once a block is found everyone agrees that you now own those coins, so you can spend them again. Until then it's possible that some network nodes believe otherwise, if somebody is attempting to defraud the system by reversing a transaction. The more confirmations a transaction has, the less risk there is of a reversal. Only 6 blocks or 1 hour is enough to make reversal computationally impractical. This is dramatically better than credit cards which can see chargebacks occur up to three months after the original transaction!

Ten minutes was specifically chosen by [[Satoshi]] as a tradeoff between first confirmation time and the amount of work wasted due to chain splits. After a block is mined, it takes time for other miners to find out about it, and until then they are actually competing against the new block instead of adding to it. If someone mines another new block based on the old block chain, the network can only accept one of the two, and all the work that went into the other block gets wasted. For example, if it takes miners 1 minute on average to learn about new blocks, and new blocks come every 10 minutes, then the overall network is wasting about 10% of its work. Lengthening the time between blocks reduces this waste.

[[File:TransactionConfirmationTimesExample.PNG]]

=== Do you have to wait until my transactions are confirmed in order to buy or sell things with Bitcoin? ===

YES, you do, IF the transaction is non-recourse. The Bitcoin reference software does not display transactions as confirmed until six blocks have passed (confirmations). As transactions are burred in the chain they become increasingly non-reversible but are very reversible before the first confirmation. Two to six confirmations are recommended for non-recourse situations depending on the value of the transactions involved.

When people ask this question they are usually thinking about applications like supermarkets. This generally is a recourse situation: if somebody tries to double-spend on a face-to-face transaction it might work a few times, but probabalistically speaking eventually one of the double-spends will get noticed, and the penalty for shoplifting charges in most localities is calibrated to be several times worse than the proceeds of a single shoplifting event.

Double-spends might be a concern for something like a snack machine in a low-traffic area with no nearby security cameras. Such a machine shouldn't honor 0-confirmation payments, and should instead use some other mechanism of clearing Bitcoin or validating transactions against reversal, see the wiki article [[Myths#Point_of_sale_with_bitcoins_isn.27t_possible_because_of_the_10_minute_wait_for_confirmation|here]] for alternatives.

Applications that require immediate payment processing, like supermarkets or snack machines, need to manage the risks. Here is one way to reverse an unconfirmed payment:

A [[Double-spending#Finney_attack|Finney attack]], in which an attacker mines a block containing a movement of some coins back to themselves. Once they find a block solution, they quickly go to a merchant and make a purchase, then broadcast the block, thus taking back the coins. This attack is a risk primarily for goods that are dispatched immediately, like song downloads or currency trades. Because the attacker can't choose the time of the attack, it isn't a risk for merchants such as supermarkets where you can't choose exactly when to pay (due to queues, etc). The attack can fail if somebody else finds a block containing the purchasing transaction before you release your own block, therefore, merchants can reduce but not eliminate the risk by making purchasers wait some length of time that's less than a confirm.

Because pulling off this attack is not trivial, merchants who need to sell things automatically and instantly are most likely to just price the cost of reversal fraud in, or use insurance.

=== I was sent some bitcoins and they haven't arrived yet! Where are they? ===

Don't panic! There are a number of reasons why your bitcoins might not show up yet, and a number of ways to diagnose them.

The latest version of the Bitcoin-Qt client tells you how far it has yet to go in downloading the blockchain. Hover over the icon in the bottom right corner of the client to learn your client's status.

If it has not caught up then it's possible that your transaction hasn't been included in a block yet.

You can check pending transactions in the network by going [http://blockchain.info here] and then searching for your address. If the transaction is listed here then it's a matter of waiting until it gets included in a block before it will show in your client.

If the transaction is based on a coin that was in a recent transaction then it could be considered a low priority transaction. Transfers can take longer if the transaction fee paid was not high enough. If there is no fee at all the transfer can get a very low priority and take hours or even days to be included in a block.

=== Why does my Bitcoin address keep changing? ===

Whenever the address listed in "Your address" receives a transaction, Bitcoin replaces it with a new address. This is meant to encourage you to use a new address for every transaction, which enhances [[anonymity]]. All of your old addresses are still usable: you can see them in ''Settings -> Your Receiving Addresses''.

===How much will the transaction fee be?===

Some transactions might require a [[transaction fee]] for them to get confirmed in a timely manner. The transaction fee is processed by and received by the bitcoin miner. The most recent version of the Bitcoin client will estimate an appropriate fee when a fee might be required.

The fee is added to the payment amount. For example, if you are sending a 1.234 BTC payment and the client requires a 0.0005 BTC fee, then 1.2345 BTC will be subtracted from the wallet balance for the entire transaction and the address for where the payment was sent will receive a payment of 1.234 BTC.

A fee might be imposed because your transaction looks like a denial of service attack to the Bitcoin system. For example, it might be burdensome to transmit or it might recycle Bitcoins you recently received. The wallet software attempts to avoid generating burdensome transactions, but it isn't always able to do so: The funds in your wallet might be new or composed of many tiny payments.

Because the fee is related to the amount of data that makes up the transaction and not to the amount of Bitcoins being sent, the fee may seem extremely low (0.0005 BTC for a 1,000 BTC transfer) or unfairly high (0.004 BTC for a 0.02 BTC payment, or about 20%). If you are receiving tiny amounts (''e.g.'' as small payments from a mining pool) then fees when sending will be higher than if your activity follows the pattern of conventional consumer or business transactions.

As of Bitcoin 0.5.3 the required fee will not be higher than 0.05 BTC. For most users there is usually no required fee at all. If a fee is required it will most commonly be 0.0005 BTC.

=== What happens when someone sends me a bitcoin but my computer is powered off? ===

Bitcoins are not actually "sent" to your wallet; the software only uses that term so that we can use the currency without having to learn new concepts. Your wallet is only needed when you wish to spend coins that you've received.

If you are sent coins when your wallet client program is not running, and you later launch the wallet client program, the coins will eventually appear as if they were just received in the wallet. That is to say, when the client program is started it must download blocks and catch up with any transactions it did not already know about.

=== How long does "synchronizing" take when the Bitcoin client is first installed? What's it doing? ===

The popular Bitcoin client software from bitcoin.org implements a "full" Bitcoin node: It can carry out all the duties of the Bitcoin P2P system, it isn't simply a "client". One of the principles behind the operation of full Bitcoin nodes is that they don't assume that the other participants have followed the rules of the Bitcoin system. During synchronization, the software is processing historical Bitcoin transactions and making sure for itself that all of the rules of the system have been correctly followed.

In normal operation, after synchronizing, the software should use a hardly noticeable amount of your computer's resources.

When the wallet client program is first installed, its initial validation requires a lot of work from your computer's hard disk, so the amount of time to synchronize depends on your disk speed and, to a lesser extent, your CPU speed. It can take anywhere from a few hours to a day or so. On a slow computer it could take more than 40 hours of continuous synchronization, so check your computer's power-saving settings to ensure that it does not turn its hard disk off when unattended for a few hours. You can use the Bitcoin software during synchronization, but you may not see recent payments to you until the client program has caught up to the point where those transactions happened.

If you feel that this process takes too long, you can download a pre-synchronized blockchain from [http://eu2.bitcoincharts.com/blockchain/ http://eu2.bitcoincharts.com/blockchain/]. Alternatively, you can try an alternative "lite" client such as Multibit or a super-light client like electrum, though these clients have somewhat weaker security, are less mature, and don't contribute to the health of the P2P network.

==Networking==
=== Do I need to configure my firewall to run Bitcoin? ===

Bitcoin will connect to other nodes, usually on TCP port 8333. You will need to allow outgoing TCP connections to port 8333 if you want to allow your Bitcoin client to connect to many nodes. [[Testnet]] uses TCP port 18333 instead of 8333.

If you want to restrict your firewall rules to a few IPs, you can find stable nodes in the [[Fallback Nodes|fallback nodes list]].

=== How does the peer finding mechanism work? ===

Bitcoin finds peers primarily by forwarding peer announcements within its own network and each node saves a database of peers that it's aware of, for future use. In order to bootstrap this process Bitcoin needs a list of initial peers, these can be provided manually but normally it obtains them by querying a set of DNS domain names which have automatically updated lists, if that doesn't work it falls back to a built-in list which is updated from time to time in new versions of the software. There is also an IRC based mechanism but it is disabled by default.

==Mining==
===What is mining?===
[[Mining]] is the process of spending computation power to secure Bitcoin transactions against reversal and introducing new Bitcoins to the system.

Technically speaking, mining is the calculation of a [[hash]] of the a block header, which includes among other things a reference to the previous block, a hash of a set of transactions and a [[nonce]]. If the hash value is found to be less than the current [[target]] (which is inversely proportional to the [[difficulty]]), a new block is formed and the miner gets the newly generated Bitcoins (25 per block at current levels). If the hash is not less than the current target, a new nonce is tried, and a new hash is calculated. This is done millions of times per second by each miner.

===Is mining used for some useful computation?===
The computations done when mining are internal to Bitcoin and not related to any other distributed computing projects. They serve the purpose of securing the Bitcoin network, which is useful.

===Is it not a waste of energy?===
Spending energy on creating and securing a free monetary system is hardly a waste. Also, services necessary for the operation of currently widespread monetary systems, such as banks and credit card companies, also spend energy, arguably more than Bitcoin would.

===Why don't we use calculations that are also useful for some other purpose?===
To provide security for the Bitcoin network, the calculations involved need to have some [http://bitcoin.stackexchange.com/questions/5617/why-are-bitcoin-calculation-useless/5618#5618 very specific features]. These features are incompatible with leveraging the computation for other purposes.

===How can we stop miners from creating zero transaction blocks?===
The incentive for miners to include transactions is in the fees that come along with them. If we were to implement some minimum number of transactions per block it would be trivial for a miner to create and include transactions merely to surpass that threshold. As the network matures, the block reward drops, and miners become more dependent on transactions fees to pay their costs, the problem of zero transaction blocks should diminish over time.

===How does the proof-of-work system help secure Bitcoin?===
To give a general idea of the mining process, imagine this setup:

payload = <some data related to things happening on the Bitcoin network>
nonce = 1
hash = [http://en.wikipedia.org/wiki/SHA2 SHA2]( [http://en.wikipedia.org/wiki/SHA2 SHA2]( payload + nonce ) )

The work performed by a miner consists of repeatedly increasing "nonce" until
the hash function yields a value, that has the rare property of being below a certain
target threshold. (In other words: The hash "starts with a certain number of zeroes",
if you display it in the fixed-length representation, that is typically used.)

As can be seen, the mining process doesn't compute anything special. It merely
tries to find a number (also referred to as nonce) which - in combination with the payload -
results in a hash with special properties.

The advantage of using such a mechanism consists of the fact, that it is very easy to check a result: Given the payload and a specific nonce, only a single call of the hashing function is needed to verify that the hash has the required properties. Since there is no known way to find these hashes other than brute force, this can be used as a "proof of work" that someone invested a lot of computing power to find the correct nonce for this payload.

This feature is then used in the Bitcoin network to secure various aspects. An attacker
that wants to introduce malicious payload data into the network, will need to do the
required proof of work before it will be accepted. And as long as honest miners have more
computing power, they can always outpace an attacker.

Also see [http://en.wikipedia.org/wiki/Hashcash Hashcash] and [http://en.wikipedia.org/wiki/Proof-of-work_system Proof-of-work system] and [http://en.wikipedia.org/wiki/SHA2 SHA2] and on Wikipedia.

===Why was the "Generate coin" option of the client software removed?===

In the early days of Bitcoin, it was easy for anyone to find new blocks using standard CPUs. As more and more people started mining, the [[difficulty]] of finding new blocks has greatly increased to the point where the average time for a CPU to find a single block can be many years. The only cost-effective method of [[Mining|mining]] is using a high-end graphics card with special software (see also [[Why a GPU mines faster than a CPU]]) and/or joining a [[Bitcoin Pool|mining pool]]. Since solo CPU mining is essentially useless, it was removed from the GUI of the Bitcoin software.

==Security==

===Could miners collude to give themselves money or to fundamentally change the nature of Bitcoin?===

There are two questions in here. Let's look at them separately.

;Could miners gang up and give themselves money?

Mining itself is the process of creating new blocks in the block chain. Each block contains a list of all the transactions that have taken place across the entire Bitcoin network since the last block was created, as well as a hash of the previous block. New blocks are 'mined', or rather, generated, by Bitcoin clients correctly guessing sequences of characters in codes called 'hashes,' which are created using information from previous blocks. Bitcoin users may download specialized 'mining' software, which allows them to dedicate some amount of their processing power – however large or small – to guessing at strings within the hash of the previous block. Whoever makes the right guess first, thus creating a new block, receives a reward in Bitcoins.

The block chain is one of the two structures that makes Bitcoin secure, the other being the public-key encryption system on which Bitcoin trade is based. The block chain assures that not only is every single transaction that ever takes place recorded, but that every single transaction is recorded on the computer of anyone who chooses to store the relevant information. Many, many users have complete records of every transaction in Bitcoins history readily available to them at any point, and anyone who wants in the information can obtain it with ease. These things make Bitcoin very hard to fool.

The Bitcoin network takes considerable processing power to run, and since those with the most processing power can make the most guesses, those who put the most power toward to sustaining the network earn the most currency. Each correct guess yields, at present, twenty-five Bitcoins, and as Bitcoins are presently worth something (although the value still fluctuates) every miner who earns any number of Bitcoins makes money. Some miners pull in Bitcoins on their own; and some also join or form pools wherein all who contribute earn a share of the profits.

Therefore, first answer is a vehement “yes” – not only can miners collude to get more money, Bitcoin is designed to encourage them to do so. Bitcoin pools are communal affairs, and there is nothing dishonest or underhanded about them.

Of course, the real question is:

;Can they do so in ways not sanctioned by Bitcoin developers? Is there any way to rip off the network and make loads of money dishonestly?

Bitcoin isn't infallible. It can be cheated, but doing so is extremely difficult. Bitcoin was designed to evade some of the central problems with modern currencies – namely, that their trustworthiness hinges upon that of people who might not have users' best interests in mind. Every currency in the world (other than Bitcoin) is controlled by large institutions who keep track of what's done with it, and who can manipulate its value. And every other currency has value because people trust the institutions that control them.

Bitcoin doesn't ask that its users trust any institution. Its security is based on the cryptography that is an integral part of its structure, and that is readily available for any and all to see. Instead of one entity keeping track of transactions, the entire network does, so Bitcoins are astoundingly difficult to steal, or double-spend. Bitcoins are created in a regular and predictable fashion, and by many different users, so no one can decide to make a whole lot more and lessen their value. In short, Bitcoin is designed to be inflation-proof, double-spend-proof and completely distributed.

Nonetheless, there are a few ways that one can acquire Bitcoins dishonestly. Firstly, one can steal private keys. Key theft isn't something that Bitcoin security has been designed to prevent: it's up to users to keep theirs safe. But the cryptography is designed so that it is completely impossible to deduce someone's private key from their public one. As long as you keep your private key to yourself, you don't have much to worry about. Furthermore, one could theoretically create a new block chain, but due to the way in which the block chain is constructed, this would be extremely difficult and require massive amounts of processing power. A full explanation of the difficulties involved can be found in the [[block chain]] article.

Bitcoin can be ripped off – but doing so would be extremely hard and require considerable expertise and a staggering amount of processing power. And it's only going to get harder with time. Bitcoin isn't impenetrable, but it's close enough to put any real worries in the peripherals.

;Could miners fundamentally change the nature of Bitcoin?

Once again, almost certainly not.

Bitcoin is a distributed network, so any changes implemented to the system must be accepted by all users. Someone trying to change the way Bitcoins are generated would have to convince every user to download and use their software – so the only changes that would go through are those that would be equally benefit all users.

And thus, it is more or less impossible for anyone to change the function of Bitcoin to their advantage. If users don't like the changes, they won't adopt them, whereas if users do like them, then these will help everyone equally. Of course, one can conceive of a situation where someone manages to get a change pushed through that provides them with an advantage that no one notices, but given that Bitcoin is structurally relatively simple, it is unlikely that any major changes will go through without someone noticing first.

The fact that such changes are so difficult to make testifies to the fully distributed nature of Bitcoin. Any centrally controlled currency can be modified by its central agency without the consent of its adherents. Bitcoin has no central authority, so it changes only at the behest of the whole community. Bitcoins development represents a kind of collective evolution; the first of its kind among currencies.

==Help==
===I'd like to learn more. Where can I get help?===

* Read the [[Introduction|introduction to bitcoin]]
* See the videos, podcasts, and blog posts from the [[Press]]
* Read and post on the [[:Bitcoin:Community_portal#Bitcoin_Community_Forums|forums]]
* Chat on one of the [[:Bitcoin:Community_portal#IRC_Chat|Bitcoin IRC]] channels
* Listen to [http://omegataupodcast.net/2011/03/59-bitcoin-a-digital-decentralized-currency/ this podcast], which goes into the details of how bitcoin works
* Ask questions on the [http://bitcoin.stackexchange.com Bitcoin Stack Exchange]

==See Also==

* [[Man page]]
* [[Introduction]]

[[de:FAQ]]
[[zh-cn:FAQ]]
[[fr:FAQ]]
[[ru:FAQ]]

[[Category:Technical]]
[[Category:Vocabulary]]

Simplified Payment Verification

2013-06-26T21:43:04Z

Oconnor663: Redirected page to Scalability#Simplified payment verification

#REDIRECT [[Scalability#Simplified_payment_verification]]

Scalability

2013-06-26T21:42:59Z

Oconnor663: note about redirect

The core Bitcoin network can scale to much higher transaction rates than are seen today, assuming that nodes in the network are primarily running on high end servers rather than desktops. Bitcoin was designed to support lightweight clients that only process small parts of the block chain (see ''simplified payment verification'' below for more details on this). A configuration in which the vast majority of users sync lightweight clients to more powerful backbone nodes is capable of scaling to millions of users and tens of thousands of transactions per second.

==Scalability targets==

VISA handles on average around 2,000 transactions per second (tps), so call it a daily peak rate of 4,000 tps. They have burst capacity for over 10,000 tps which they need to handle the busiest points of the holiday period (~8,500 tps). [http://investor.visa.com/phoenix.zhtml?c=215693&p=irol-newsArticle_print&ID=1355716]

PayPal, in contrast, handles around 4 million transactions per day for an average of 46 tps or a probably peak rate of 100 tps.

Let's take 4,000 tps as starting goal. Obviously if we want Bitcoin to scale to all economic transactions worldwide, including cash, it'd be a lot higher than that, perhaps more in the region of a few hundred thousand tps. And the need to be able to withstand DoS attacks (which VISA does not have to deal with) implies we would want to scale far beyond the standard peak rates. Still, picking a target let us do some basic calculations even if it's a little arbitrary.

==Current bottlenecks==

Today the Bitcoin network is restricted to a sustained rate of 7 tps by some artificial limits. These were put in place to stop people from ballooning the size of the block chain before the network and community was ready for it. Once those limits are lifted, the maximum transaction rate will go up significantly.

==CPU==

The protocol has two parts. Nodes send "inv" messages to other nodes telling them they have a new transaction. If the receiving node doesn't have that transaction it requests it with a getdata.

The big cost is the crypto and block chain lookups involved with verifying the transaction. Verifying a transaction means some hashing and some ECDSA signature verifications. RIPEMD-160 runs at 106 megabytes/sec (call it 100 for simplicity) and SHA256 is about the same. So hashing 1 megabyte should take around 10 milliseconds and hashing 1 kilobyte would take 0.01 milliseconds - fast enough that we can ignore it.

Bitcoin is currently able (with a couple of simple optimizations that are prototyped but not merged yet) to perform around 8000 signature verifications per second on an quad core [http://ark.intel.com/products/53469 Intel Core i7-2670QM 2.2Ghz processor]. The average number of inputs per transaction is around 2, so we must halve the rate. This means 4000 tps is easily achievable CPU-wise with a single fairly mainstream CPU.

As we can see, this means as long as Bitcoin nodes are allowed to max out at least 4 cores of the machines they run on, we will not run out of CPU capacity unless Bitcoin is handling 100 times as much traffic as PayPal. As of late 2012 the network is handling 0.5 transactions/second, so even assuming enormous growth in popularity we will not reach this level for a long time.

==Network==

Let's assume an average rate of 2000tps, so just VISA. Transactions vary in size from about 0.2 kilobytes to over 1 kilobyte, but it's averaging half a kilobyte today.

That means that you need to keep up with around 8 megabits/second of transaction data (2000tps * 512 bytes) / 1024 bytes in a kilobyte / 1024 kilobytes in a megabyte = 0.97 megabytes per second * 8 = 7.8 megabits/second.

This sort of bandwidth is already common for even residential connections today, and is certainly at the low end of what colocation providers would expect to provide you with.

When blocks are solved, the current protocol will send the transactions again, even if a peer has already seen it at broadcast time. Fixing this to make blocks just list of hashes would resolve the issue and make the bandwidth needed for block broadcast negligable. So whilst this optimization isn't fully implemented today, we do not consider block transmission bandwidth here.

==Storage==

At very high transaction rates each block can be over half a gigabyte in size.

It is not required for most fully validating nodes to store the entire chain. In Satoshi's paper he describes "pruning", a way to delete unnecessary data about transactions that are fully spent. This reduces the amount of data that is needed for a fully validating node to be only the size of the current unspent output size, plus some additional data that is needed to handle re-orgs. As of October 2012 (block 203258) there have been 7,979,231 transactions, however the size of the unspent output set is less than 100MiB, which is small enough to easily fit in RAM for even quite old computers.

Only a small number of archival nodes need to store the full chain going back to the genesis block. These nodes can be used to bootstrap new fully validating nodes from scratch but are otherwise unnecessary.

The primary limiting factor in Bitcoin's performance is disk seeks once the unspent transaction output set stops fitting in memory. It is quite possible that the set will always fit in memory on dedicated server class machines, if hardware advances faster than Bitcoin usage does.

==Optimizations==

The description above applies to the current software with only minor optimizations assumed (the type that can and have been done by one man in a few weeks).

However there is potential for even greater optimizations to be made in future, at the cost of some additional complexity.

===CPU===

Pieter Wuille has implemented a custom verifier tuned for secp256k1 that can go beyond 20,000 signature verifications per second. It would require careful review by professional cryptographers to gain as much confidence as OpenSSL, but this can easily halve CPU load.

Newly developed digital signature algorithms, like [http://ed25519.cr.yp.to/ed25519-20110705.pdf ed25519] have extremely efficient software implementations that can reach speeds of nearly 80,000 verifications per second, even on an old Westmere CPU. That is a 10x improvement over secp256k1, and most likely is even higher on more modern CPUs. Supporting this in Bitcoin would mean a "soft fork" like the one done recently for pay-to-script-hash support.

Algorithms exist to accelerate batch verification over elliptic curve signatures. This means if there are several inputs that are signing with the same key, it's possible to check their signatures simultaneously for another 9x speedup. This is a somewhat more complex implementation, however, it can work with existing signatures (clients don't need upgrading).

Assuming no upgrades to lightweight/SPV clients, so just batch verification, we can reach 40,000 transactions per second which is far beyond the traffic levels of the entire credit card system. And with ed25519 we could go up to nearly half a million transactions per second - with todays hardware! At these speeds it is likely that bandwidth to disk would become the primary limiting factor.

===Simplified payment verification===


It's possible to build a Bitcoin implementation that does not verify everything, but instead relies on either connecting to a trusted node, or puts its faith in high difficulty as a proxy for proof of validity. [[bitcoinj]] is an implementation of this mode.

In Simplified Payment Verification (SPV) mode, named after the section of Satoshi's paper that describes it, clients connect to an arbitrary full node and download only the block headers. They verify the chain headers connect together correctly and that the difficulty is high enough. They then request transactions matching particular patterns from the remote node (ie, payments to your addresses), which provides copies of those transactions along with a Merkle branch linking them to the block in which they appeared. This exploits the Merkle tree structure to allow proof of inclusion without needing the full contents of the block.

As a further optimization, block headers that are buried sufficiently deep can be thrown away after some time (eg, you only really need to store say 5000 blocks).

The level of difficulty required to obtain confidence the remote node is not feeding you fictional transactions depends on your threat model. If you are connecting to a node that is known to be reliable, the difficulty doesn't matter. If you want to pick a random node, the cost for an attacker to mine a block sequence containing a bogus transaction should be higher than the value to be obtained by defrauding you. By changing how deeply buried the block must be, you can trade off confirmation time vs cost of an attack.

Programs implementing this approach can have fixed storage/network overhead in the null case of no usage, and resource usage proportional to received/sent transactions.

See also: [[Thin Client Security]].

[[Category:Technical]]