English Bahasa Indonesia Dansk Deutsch English Español Français Italiano Magyar Nederlands Polski Português Brasil Română Slovenščina Svenska Türkçe Ελληνικά български Русский Українська العربية فارسی हिन्दी 한국의 日本語 简体中文 繁體中文 Bahasa Indonesia Dansk Deutsch English Español Français Italiano Magyar Nederlands Polski Português Brasil Română Slovenščina Svenska Türkçe Ελληνικά български Русский Українська العربية فارسی हिन्दी 한국의 日本語 简体中文 繁體中文 51% Attack, Majority Hash Rate Attack This definition comes from the technical glossary.Definition The ability of someone controlling a majority of network hash rate to revise transaction history and prevent new transactions from confirming.

Synonyms 51 percent attack Majority attack Developer Guide What can an attacker with 51% of hash power do?— Bitcoin StackExchange )Jump to: , Contents 1 2 3 4 The wallet is stored unencrypted, by default, and thus becomes a valuable target for theft.Recent releases of the Bitcoin client now supports encryption to protect the wallet data, though the user must opt-in.An old copy of a wallet with its old password is often easily retrievable via an existing backup facility (particularly Apple Time-Machine): draining that old wallet, with its old password, drains the current wallet with the current password -- this is contrary to most non-technical users expectation of what 'change the password on your wallet' should mean following password compromise.An initial solution is to mandate (either in code or as expressed policy) that changing a wallet's password causes (or asks the user to cause) the creation of a new wallet with new addresses, and the sending of existing sums to them.

Backed-up copies of the original wallet with the original password would then be empty, should they be compromised.On the downside, the password-changing process would potentially take much longer, cost a transaction fee or more, and - intially at least - the new wallet is no longer backed up.bitcoin aiutoOn the upside, non-technical users won't find their wallets drained from security compromises they believed they had closed, nor be required to locate existing backups of a wallet in order to destroy them.trade bitcoin with mt4Tracing a coin's history can be used to connect identities to addresses.bitcoin reitAn attacker can attempt to fill the network with clients controlled by him, you would then be very likely to connect only to attacker nodes.third party bitcoin wallet

Although Bitcoin never uses a count of nodes for anything completely isolating a node from the honest network can be helpful in the execution of other attacks.This state can be exploited in (at least) the following ways: Bitcoin makes these attacks more difficult by only making an outbound connection to one IP address per /16 (x.y.0.0).bitcoin difficulty growth rateIncoming connections are unlimited and unregulated, but this is generally only a problem in the anonymity case, where you're probably already unable to accept incoming connections.bitcoin offline backupLooking for suspiciously low network hash-rates may help prevent the second one.Someone who can see all of your Internet traffic can easily see when you send a transaction that you didn't receive (which suggests you originated it).Bitcoin-QT has good Tor integration which closes this attack vector if used.

Sending lots of data to a node may make it so busy it cannot process normal Bitcoin transactions.Bitcoin has some denial-of-service prevention built-in, but is likely still vulnerable to more sophisticated denial-of-service attacks.These are the current Bitcoin Satoshi client protections to deter DoS attacks, as of version 0.7.0: These are protocol rules built to prevent DoS: These are the Satoshi client protections added in version 0.8.0: Satoshi client does not directly limit peer bandwidth nor CPU usage.See Timejacking for a description of this attack.It can be fixed by changing how nodes calculate the current time.It is illegal in some countries to possess/distribute certain kinds of data.Since arbitrary data can be included in Bitcoin transactions, and full Bitcoin nodes must normally have a copy of all unspent transactions, this could cause legal problems.However, Local node policy generally doesn't permit arbitrary data (transactions attempting to embed data re non-standard), but steganographic embedding can still be used though this generally limits storage to small amounts.

Various ideas have been proposed to further limit datastorage in the UTXO set but are not currently being seriously considered for deployment.It's possible but unlikely that a newly discovered bug or security vulnerability in the standard client could lead to a block chain split, or the need for every node to upgrade in a short time period.For example, a single malformed message tailored to exploit a specific vulnerability, when spread from node to node, could cause the whole network to shutdown in a few hours.Bugs that break user anonymity, on the contrary, have been found, since the pseudo-anonymity property of Bitcoin has been analyzed less.Starting from version 0.7.0, Bitcoin client can be considered a mature project.The security critical sections of the source code are updated less and less frequently and those parts have been reviewed by many computer security experts.Also Bitcoin Satoshi client has passed the test of being on-line for more than 3 years, without a single vulnerability being exploited in the wild.

See Common Vulnerabilities and Exposures for a detailed list of vulnerabilities detected and fixed.Energy consumption for mining has a high correlation with bitcoin value (exchange rate).Because variable costs of mining are dominated by electricity price, the economic equilibrium for the mining rate is reached when global electricity costs for mining approximate the value of mining reward plus transaction fees.So the higher the value of one bitcoin, the higher the value of mining rewards and transaction fees, the higher the energy consumption of the bitcoin network in the long run.SHA-256 and ECDSA are considered very strong currently, but they might be broken in the far future.If that happens, Bitcoin can shift to a stronger algorithm.Bitcoin can easily scale beyond the level of traffic VISA sees globally today.See the discussion on the scalability page for more information.If there is even a "trickle" of a connection between two sides of a segmented network, things should still work perfectly.

When block chains are combined, all of the non-generation transactions in the shorter chain are re-added to the transaction pool -- they'll start over at 0/unconfirmed, but they'll still be valid.No mature transactions will be lost unless the segmentation persists for longer than ~120 blocks.Then generations will start to mature, and any transactions based on those generations will become invalid when recombined with the longer chain.The IP addresses of most users are totally public.You can use Tor to hide this, but the network won't work if everyone does this.Bitcoin requires that some country is still free.Nodes that generate blocks can choose not to include a transaction in their blocks.When this happens, the transaction remains "active" and can be included in a later block.Two things discourage this: An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions.This allows him to: Note that the above limitations only apply to the perspective of Bitcoin as seen by full nodes.

Some lightweight nodes work by trusting miners absolutely; from the perspective of Bitcoin as seen by lightweight nodes, miners can steal BTC, etc. This is one of the reasons why lightweight nodes are less secure than full nodes.With less than 50%, the same kind of attacks are possible, but with less than 100% rate of success.For example, someone with only 40% of the network computing power can overcome a 6-deep confirmed transaction with a 50% success rate [1].It's much more difficult to change historical blocks, and it becomes exponentially more difficult the further back you go.As above, changing historical blocks only allows you to exclude and change the ordering of transactions.If miners rewrite historical blocks too far back, then full nodes with pruning enabled will be unable to continue, and will shut down; the network situation would then probably need to be untangled manually (eg.by updating the software to reject this chain even though it is longer).Since this attack doesn't permit all that much power over the network, it is expected that rational miners will not attempt it.

A profit-seeking miner should always gain more by just following the rules, and even someone trying to destroy the system might find other attacks more attractive.Probably the most likely scenario where this attack would be employed would be for a government to try to get control over Bitcoin by acquiring a majority of hashing power (either directly or by enforcing rules on private miners within its borders).Then this government could use the transaction-censorship power listed above to do things like: The appropriate response to any long-term attack by miners is a hardfork to change the proof-of-work function.This fires all existing miners, and allows totally new ones to replace them.It is easy to send transactions to yourself repeatedly.If these transactions fill blocks to the maximum size (1MB), other transactions would be delayed until the next block.This is made expensive by the fees that would be required after the 50KB of free transactions per block are exhausted.An attacker will eventually eliminate free transactions, but Bitcoin fees will always be low because raising fees above 0.01 BTC per KB would require spending transaction fees.

An attacker will eventually run out of money.Even if an attacker wants to waste money, transactions are further prioritized by the time since the coins were last spent, so attacks spending the same coins repeatedly are less effective.Named for Hal Finney, who first described this variation of a double-spend attack involving accepting 0-confirmation transactions.Accepting 0-confirmation large-value transactions is problematic; accepting them for low-value transactions (after waiting several seconds to detect an ordinary double-spend attempt) is probably safe.Any rival client must follow Bitcoin's rules or else all current Bitcoin clients will ignore it.You'd have to actually get people to use your client.A better client that pretends to follow the same rules, but with an exception known only to the author (possibly by making it closed source), might conceivably be able to gain widespread adoption.At that point, its author could use his exception and go largely unnoticed.Bitcoin has 2.1 quadrillion raw units, making up 8 decimals of BTC precision, so the entire network could potentially operate on much less than the full quantity of Bitcoins.

If deflation gets to the point where transactions of more than 10 BTC are unheard of, clients can just switch to another unit so that, for example, it shows 10 mBTC rather than 0.01 BTC.The maximum number of raw units might not be enough if the entire world starts using BTC, but it would not be too difficult to increase precision in that case.The transaction format and version number would be scheduled to change at some particular block number after a year or two, and everyone would have to update by then.Generating an address doesn't touch the network at all.You'd only be wasting your CPU resources and disk space.Also, a collision is highly unlikely.Keys are 256 bit in length and are hashed in a 160 bit address.(2^160thpower) Divide it by the world population and you have about 215,000,000,000,000,000,000,000,000,000,000,000,000 addresses per capita.(2.15x 10^38)[1] If everyone began with identical blocks and started their nonce at 1 and incremented, the fastest machine would always win.